You just need to prepare and submit self assessment tax returns for each financial/tax year. That basically means entering the totals of your business income, and business expenses, which produces your taxable profit, which produces the tax/NIC liability. (Plus you need to enter anything else for each year such as wages from elsewhere, interest received, dividends received, pensions, gift aid payments, etc etc, all for each full tax year).
So, you need to keep some book-keeping records which will record each transaction (in and out), and total them up for the year for each category.
You don't send HMRC your detailed book-keeping records, nor invoices, bank statements, etc., just the tax return itself with the total figures on it, but you need to have the paperwork, book-keeping records etc available to send to HMRC later if they ask for them by opening an enquiry into the tax return.
Lots of book-keepers and accountants can help you with all that if you're not confident to research and do it yourself.