Four or five things I know about Johanna Basford.

She pronounces the H in her first name. She loves tea, but not mayonnaise. She is not a big Mariah Carey fan, would love to design a Christmas window for Selfridges and, oh yes, she's sold one million copies worldwide of her colouring book Secret Garden.

Let me repeat that. One million copies. In 20 languages. In case you think everyone in publishing sells JK Rowling amounts let's just point out that one million copies is not normal. One million copies is very not normal. "When we published Secret Garden we knew people would love her illustrations as much as we did," says Eleanor Blatherwick, head of sales and marketing at her publisher Laurence King, "but could not have predicted it to be the runaway success it has been."

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"Do you know, I didn't realise we had sold a million books," Basford tells me as we sit drinking tea (naturally). "I knew we had got to 100,000 and I was really pleased about that. Just before Christmas we got to 200,000 so I phoned up Laurence King and said, 'Can you please tell me when we get to a quarter of a million because I want to do a thing on my blog and I'm really excited.' And there was a long pause at the end of the phone and they said, 'Well, actually, the foreign edition numbers have just come in and we think you're almost at a million.'"

She grins. "I can't even picture a million books. It's such a big number. Nobody is more surprised than I am."

We are sitting in her home near Ellon in Aberdeenshire with her husband James Watt, co-founder of Scottish craft beer company BrewDog. It's a cold, clear blue February afternoon. Big skies, bare trees. In here the dog is snoring. Her eight-month-old baby Evie has gone a walk with the nanny and Basford is sitting talking about art and craft and why adults as much as kids are now buying colouring in books.

"I get a lot of emails from people who are maybe in rehab or rehabilitation for illness or are still in hospital and they say it's a nice thing to do because it's calming," she says.

Well, indeed. The colouring in craze seems to have started in France where books were explicitly marketed as a de-stressing exercise and it has spread here. Waterstones has revealed there was a 300 per cent rise in sales of colouring books between Christmas 2013 and Christmas 2014. One wonders how many of those sales were Secret Garden.

"When you're in that flow you're caught up in the moment. That's how I feel when I'm drawing. People get a sense of tranquillity when they're colouring in. They would probably get the same feeling from drawing but a blank sheet of paper can be intimidating."

Not for her clearly. Basford is doing her part to keep the colouring in trend going with a new book Enchanted Forest, partly inspired by childhood visits to her grandparents at Brodick Castle on Arran where her grandfather was head gardener. She's already working on a third book too. This is part Evie-related. Since her daughter has come along, Basford has stepped back from a very hectic programme of commercial commissions to concentrate on the books.

Enchanted Forest, like its predecessor, is full of fantastical visions of flora and fauna rendered in precise and delicate black and white, all drawn by hand. "I just use the computer at the end more as a finishing tool. I usually have to edit out tea splashes, the odd chocolate smudge," their creator says.

"There are people who can do digital stuff and it's amazing. I can't and I feel that [if you're] drawing nature it makes sense to draw it by hand. It reflects the subject matter really well. And I like wobbly lines and imperfect circles. It looks natural and human. It's got a soul. Computer-generated stuff is very crisp, but it can also be quite clinical and it doesn't have the same character. There is something nice about a lopsided circle or a triangle that's not quite perfect."

Actually, she thinks, the analogue nature of colouring in is one of its other appeals. "You're not plugged in. You're not looking at emails or updates. You're not looking at Twitter. It's not a competitive sport. It's just something you can do quietly by yourself and be creative."

She could be describing her own life. Basford, 31, is an example of the ability of creatives to live local and work global. A graduate of Duncan of Jordanstone in Dundee, she's worked for clients ranging (alphabetically) from Absolut Vodka to Which?, from Tate Modern to The Body Shop, all from her home in the Aberdeenshire countryside. And of course she has done designs for BrewDog. "Yes, the pretty ones. The less aggressive, less manly, pretty ones."

She is a country girl by custom and preference. Apart from her very earliest years spent in Aberdeen itself, her student years in Dundee and a brief, unhappy spell in London, she's not chased the bright lights. The daughter of marine biologists, she grew up on her parents' freshwater fish farm pushing wheelbarrows, hauling nets and permanently in wellingtons. "My mum and dad were like total TV dictators. We didn't have a computer or anything so we were outside the whole time. It was that free-range childhood where you have to make things up and use your imagination. You're building dens and fighting monsters and guddling about in the burn. Those kind of games that I played with my sister all feed back into my work."

When she wasn't outside, she could be found with a pencil in her hand. Her parents encouraged her too. Mostly. "Drawing on the walls wasn't really encouraged. My dad had this thick black - almost tar - paint that he used to paint on the underside of the car; I'm assuming to seal it. I painted the wall with that. I think I was four. That didn't go down well. Drawing on my sister wasn't allowed either."

Fortunately she found no shortage of other things to draw on. After school she moved to Dundee where she put in the time. At Duncan of Jordanstone she would arrive with the cleaners early in the morning and not leave until 14 hours later. "I was the kind of person who thought, 'I've got four years. I'm paying to be here. I'm not going to get this opportunity again.'"

She'd opted to study printed textiles rather than illustration, mostly because the latter department was located down in the basement and didn't have a view. Her attraction to black-and-white hand drawings was partly personal choice, partly expediency. "When I was in my final year we got our first digital printer in the department. Everyone was doing digital textiles and the queue was really long for this printer so I just thought, 'I'll do screenprinting.' And I was skint so I worked out black and white was the cheapest way to screenprint."

It proved a smart move. At the end of fourth year she contributed to a graduation show in London entitled New Designers. It led to a phone call from American fashion label DKNY. "I'd never been phoned from America before. They wanted some wallpaper to hang in their store in Bond Street."

She stayed in London after graduation but she couldn't say she enjoyed it. "It was big and dirty and noisy and there was not enough green and you can't draw flowers when you're surrounded by concrete. Unfortunately, it was the same time as the 7/7 attacks. I was working in Brixton at the time and got caught up in the Stockwell tube station stuff. We were sitting on the tube and ours wouldn't move.

Everyone was on edge anyway. That happened on the Thursday morning and on the Friday night I got the Megabus home to Aberdeen. That was it. Bye bye. And I never went back ... Well, I go back for work obviously, but I would never move back."

She retreated to Dundee and continued to make printed wallpapers for "really expensive homes and luxury hotels and posh shops" which would pay her £400 for a hand-printed roll of wallpaper. "If I'm honest I probably wasn't running the business very well and not making the amount of money I needed to and I was working two part-time jobs to buffer it. I was a waitress at night - badly - and I worked in a clothes shop as well. When the recession hit that was the end of that. I had to really look at what I was doing and think, 'I'm not making the money.' So I closed the studio, sold all my stuff and focused on illustration because I figured I could work from a desk in my bedroom. No overheads. No big equipment. I would just need to buy pens and a laptop and a scanner."

Not that it was easy. "I was skint. I was up to my eyes in debt. Borrowing Pot Noodles off my mates. Thinking, 'This is grim.' Doing all the part-time jobs I could even on minimum wage. But I just thought, 'I need to make this work.' I wrote a cheque to myself for 10 grand. Forward-dated it for a year and stuck it on my fridge and said, 'Right, I need to pay myself that 10 grand.' I did my 'commission mission' as I called it. I would get on the Megabus to Edinburgh, Glasgow and London to see creative directors - the overnight bus because I'd be working through the day. It takes 12 hours from Aberdeen and Victoria Bus Station is grim at six o'clock in the morning. Get the overnight bus back. Each week. And it started to work. Really slowly but it did start to work."

Things began to pick up. And she was smart. She crowdsourced ideas to draw from Twitter and spent two days drawing it as a way of promoting her work. Did it again and streamed it live. It all helped get her known to creative directors and commissioning directors. Soon work offers came in. She was commissioned to draw the cover of the Edinburgh Fringe in 2010. "They used the artwork for the banners so all my drawings suddenly came to life. It was weirdly embarrassing. I used to get really flushed if somebody was walking past with the programme."

There's been the odd odd job too. Like the time she was asked to draw all over a Mercedes Smart car. "They got people to suggest their ideas of the future and I was drawing all this stuff. A gin and tonic making robot. A bed that could fly you to the moon.

"I spent two days drawing on this car which before I did it sounded fine but was tortuous. It was so painful sitting on the floor. A car's all funny shapes and I couldn't reach the top, even though it was a small car. So physically that was difficult. And also drawing on a car is difficult. It was like trying to draw on a banana with a biro."

People even ask her to draw designs for tattoos on to their skin for them. She used to do them but the responsibility is too great. "A tattoo is such a big investment for the person having it. They're living with your work for the rest of their life. So I don't do them any more purely because it's so stressful. The pressure to get it right is huge."

Instead she says she'll stick to the books. To be honest she'd been uncertain about doing Secret Garden when it was first suggested. She had too much commercial work to do at the time and she didn't know if a book would make any money. She even tried to pull out but her publisher twisted her arm. Just as well. With Enchanted Forest "they didn't need to persuade me".

Johanna Basford doesn't have to borrow Pot Noodles these days. So that's a happy ending then. Isn't it? She's not sure. "I'm petrified! I'm always on the cusp of fear. But I think that's good. I always think: 'I could have done that better.' And now I know the first book has sold a million the pressure's on. I don't want to be a one-hit wonder. That's my big fear. I don't want to be the Whigfield of colouring in books."

Enchanted Forest and Secret Garden are published by Laurence King, priced £9.95 each.