Every January, HMRC receives a flood of last-minute returns, and every January, thousands of them contain avoidable errors that cost real money. The truth is that Self Assessment rewards the organised, and the organisation required is smaller than most people fear.
Start with the documents: your P60 or P45, records of any self-employed income and expenses, dividend vouchers, bank interest statements, and details of any property income. Gather these in one sitting and the return becomes an exercise in transcription rather than archaeology.