Across Scotland this week, concerned citizens are penning preambles to their submissions to the Smith Commission on the constitutional future of Scotland.

Lord Smith of Games has to honour the solemn and binding vow from the UK party leaders that "extensive new powers" will be devolved if Scotland votes to stay in the UK.

Regrettably, civic Scotland has been excluded from this process, even though it was civic Scotland - through the Scottish Constitutional Convention - that has driven constitutional reform to date, not self-interested political parties. But Lord Smith has said that everyone can "have a say", so get to your keyboards people and make a submission before October 31.

It's easy: look on the website for the address and the format. But don't just cut and paste something from Wings Over Scotland and expect it to be looked at because anything pro-forma will be rejected. Put it in your own words, even if it is just to say that what you heard was a promise of "devo max". You heard it from Gordon Brown ("as near to federalism as is possible"); you heard it from the Better Together spokesman, George Galloway, at the Big Big Debate; you read it in that famous letter from JK Rowling that persuaded you to vote No; and you read it in countless newspaper editorials. You heard the Prime Minister himself say that "all things are possible".

But, more important than all that, devo max seems to be what the Scottish people want, according to Professor John Curtice in his blog post on What Scotland Thinks on September 30. All recent polls indicate that, by a margin of around two to one, the Scottish voters want all important decisions on tax, not just income tax, and welfare to be devolved to Holyrood. A similar margin want defence and foreign affairs kept at Westminster.

The polls emphasise what the political parties agree on, which is that there should be a sovereignty declaration to "entrench the Scottish Parliament". It means that the Scottish Parliament can never be abolished by an act of the UK Parliament, in the way the Greater London Council was abolished by Margaret Thatcher in 1986.

More importantly, this means that the Scottish Parliament exercises power - what "sovereignty" means - on its own account and not as delegated by Westminster. This replaces the assertion in the Scotland Act 1998 which says that "sovereignty in all matters rests with Westminster". This really could be the first line in the new UK constitution.

So, it exercises sovereignty, but what powers should Holyrood have? Well, here we invoke the most important principle of modern constitutional practice, one that all parties agree on: subsidiarity. This means, as Mr Brown put it, that only those powers that must remain at the centre do remain at the centre. The Scottish Parliament was founded on the principle of subsidiarity. Only those powers that are reserved to Westminster are specified in the Scotland Act; all the rest are with Holyrood.

So what powers must reside at Westminster? Well, to ensure the continuity of the UK, defence and foreign affairs must reside at the UK level so that it presents "a common face to the world". This doesn't mean the Scottish Parliament cannot make its views known about illegal wars or Trident, but for a state to function it must have powers over war and peace and the ability to negotiate treaties.

There must also be a common economic space, meaning a currency union with a central bank that sets interest rates for the whole Union, oversees banking regulation and sets limits on borrowing. The UK must also rule on common trading standards, weights and measures and most employment law. It must have the power to promote economic activity across the entire Union through investment in infrastructure and matters such as the electricity grid.

For there to be a UK, there must also be a common civic space such that all citizens have equal rights under protocols such as the European Convention on Human rights. They should have basic standards of decency and welfare. And the UK must have legal authority to intervene if these standards are not being met.

Subsidiarity is about accountability and taking responsibility. The Scottish Parliament must raise taxes to fund its spending on health, education, justice, welfare and so on. How it does this should largely be the business of the elected Scottish Government to decide, not Westminster politicians or unelected Lords.

The Scottish Parliament should have powers to raise income tax, national insurance, corporation tax, inheritance tax, petroleum revenue. None of these is necessary for the continuation of the Union. Levels of Value Added Tax and some excise duties cannot be set at regional/state level, and nor can external trading tariffs. But, other than that, just let it be.

The future of the United Kingdom does not rest on what percentage of income tax is raised by Holyrood. Indeed, the constant fiddling with bits of income tax here and a bit of air passenger duty there is a source of constitutional instability. The Smith Commission should not be trying to micromanage the financial affairs of future Scottish governments. If the Scottish Parliament gets it wrong, its government will be thrown out at the next election.

Fiscal autonomy has been described as "independence under another name", "a Trojan Horse", a "threat to the integrity of the UK". This is not the case. In Canada, for example, provincial governments can, in theory, tax anything they want to tax, except internal and international trade. They can set their own rates, using their own definitions of tax bases, and collect taxes themselves. And, of course, they can borrow on the national and international money markets.

Similarly, under subsidiarity, it makes little sense for matters such as abortion to be reserved to Westminster when the Scottish Parliament already has responsibility for sexual health, the justice system, family law, marriage, child welfare and so on. If Holyrood can decide on ethical matters such as gay marriage, it is quite capable of handling abortion.

Broadcasting should be devolved to Holyrood because Scotland is a nation with its own culture, political system, institutions and history. This does not mean that politicians interfere in editorial decisions any more than they do at Westminster. But it is not necessary for the maintenance of the UK for this regulatory responsibility to lie in London.

And that's about it. Joking aside, the Smith Commission can't write a new constitution for the UK in six weeks, so it should not try to deal with the West Lothian Question. That is a matter for England, Wales and Northern Ireland. Similarly, the Barnett formula has to remain in some form. All federal systems have equalisation formulae between states.

Lord Smith just needs to get sovereignty and subsidiarity right and the rest will fall into place. And listen to the people, because they usually get it right.