Obviously very different if you're self employed or have income mostly from investments or properties, but the you have an accountant to do it for you.
The income numbers are already there and if I want to check it's easy: my employer gives me a form with the same numbers in the same numbered boxes. I just need to specify how much income I had from bank interest.
The tax witholding system usually works as well - the main exception being straight after starting work for the first time or changing jobs, when you can have a temporary code. In these cases I just called HMRC and told them what was going on. The employer gives my pay numbers to HMRC and HMRC give my employer a tax code that determines how much to withhold each month.