What Self Assessment is and who it is for
Self Assessment is how HMRC collects tax on income that is not taxed before you get it. See why a sole trader sends a return.
This path is practice in how the records and the sums work. It is not tax advice. Any rate or allowance in a question is an example for that question, so check GOV.UK for the current figures.
An employee's tax is taken out of their pay before they get it. Nobody takes tax out of a sole trader's income, so the sole trader has to report it.
Self Assessment is the system for that. You send HMRC a tax return for each tax year, and the tax is worked out from the figures on it.
A sole trader is taxed on profit: business income less business expenses.
Most sole traders have to register with HMRC for Self Assessment. Whether you must register, and by when, depends on limits and dates that are on GOV.UK.