When he takes office, every Chancellor is given a letter from a colleague of Screwtape which tells him how to write Budgets. I've got a copy of this, which I reproduce below.

Dear Chancellor.

Some new Chancellors believe that Budgets are a chance from them to earn a reputation as the man who transforms the economy. Cleanse yourself of such childish nonsense. A sensible tax system, along the lines suggested by the Mirrlees review, would not need to be tinkered with every few months; taxes should be predictable. The Budget is not about the economy. It's about something much more important than that. It's about YOU.

A Budget is a pantomime in which you are the star. Like all pantomimes, it must follow a rigid structure.

Your preamble will discuss the general economic background. If you've taken over from the opposing party, you say what a mess they've left. If you've been in the job a few years, cherry-pick a few facts to make it look as if you've been a success. If the economy's weak, for example, celebrate low inflation and low mortgage rates. If it's strong, talk about jobs.

Waffle about prudence, stability and tough choices is always advised here. Make your job seem more difficult than it is.

You then discuss economic forecasts. Under no circumstances must you say that these are usually wrong. Nor must you discuss margins of error. A Budget is no place for serious economics. Instead, the forecasts allow you to talk about the global economy. This gives you gravitas. If the outlook is good, you can claim credit for your past policies. If the outlook is bad, blame your predecessor or the challenging world environment and say you must make tough choices: it's much better to look tough than mean.

You then announce a long list of measures intended to help charities, small businesses, science and new industries. These measures shouldn't involve more than a few million and won't make a lick of difference. But associating yourself with science and good causes whilst appearing generous will make you look good.

Also, announce a few infrastructure projects. Journalists are wise now to the Yes Minister theory of infrastructure spending, so you can't do anything about the A34/M40 junction. Broadband and something called "the North" are popular, however.

Oh, and don't forget to bugger about with North Sea oil taxation. It's an annual tradition.

Promise to raise a few billions by making efficiency savings and by clamping down on tax avoidance and loopholes. Skimp on the details, though. And if you've been in office a few years, never explain why you didn't make these "savings" earlier. You can probably also get away with raising money by taxing banks or utilities. Nobody likes them.

You might also want to raise money in other ways, such as by welfare cuts. Ensure that you do so in the most complicated way you can. And never mention the sums involved. The great thing about a Budget is that you can get away with a complete lack of proportion. If you announce a litany of tiny giveaways followed by a complicated change to tax credits or welfare benefits you can appear generous even if you're not.

Of course, policy wonks will spot this. But by the time they do so, the public will have gotten tired of Budget coverage so they won't care. It's the initial headlines that matter, and these are provided by gullible political journalists who only hear the speech and don't understand numbers.

Next, take a penny off a pint. It makes Radio 5 Live listeners happy.

Then it's time for personal taxes. Raise the personal allowance. This allows you to boast of taking low-paid people out of tax. The main beneficiaries will of course be higher earners. So you can look as if you're helping the worst off whilst in fact benefiting the median voter. It's a double win.

Then move onto savings. You want to "encourage savings", so cut taxes on these: your civil servants might have spoken about income and substitution effects, but if you're wise you'll have ignored them.

Finally, there's the rabbit from the hat. Always end your speech with a surprise giveaway.

Above all, remember what the Budget is all about. It's about how you appear in newspaper headlines the following day. Nothing else matters.