If you are completing the 16/17 tax return, the paper deadline passed on 31/10/17 and to avoid any penalties you would be completing the tax return online by 31/01/18. The online return will ask you if you used the 'cash basis' which describes your situation. You can then enter your turnover and expenses, either as a single value or detailed breakdown, to calculate your profit/loss. Access and completion of the online tax return is the same for all cases.
If this is about the paper return and you have an extended deadline or this query is for next year, you will be able to complete the SA103S Self-employment (short) as part of your full return, rather than the SA103F Self-employment (full) which asks for a more detailed breakdown of a business.