Self Assessment stress isn't caused by the tax return itself — it's caused by leaving everything to January. A little preparation in the autumn transforms the whole process.
Start by gathering everything income-related: employment P60s and P45s, self-employed income summaries, dividends, interest statements, rental income and any capital disposals.
Next, expenses. If you're self-employed, categorise by month rather than by supplier — it makes review easier. If you use software already, most of this is done.
Don't forget the reliefs: pension contributions, Gift Aid, marriage allowance, professional subscriptions, EIS/SEIS. These are where accountants pay for themselves.
File early, pay by 31 January. Filing early doesn't mean paying early — you still have until the deadline to pay, but you have months of certainty about how much.