Registering as a sole trader means registering for Self Assessment with HMRC as self-employed. You must do it if you earned more than GBP 1,000 from self-employment in a tax year, by the 5 October after that tax year ends. It is free, done online, and gives you a Unique Taxpayer Reference (UTR).
Who must register - Anyone earning more than GBP 1,000 from self-employment in a tax year (6 April...
Deadline - By 5 October after the end of the tax year you started trading
Cost - Free - there is no fee, unlike a limited company
How - Online through your HMRC Government Gateway account
Plus 6 more in the full guide

freedomisntfree.co.uk/guides/register-as-sole-trader-uk