The first and most important step is to advise HMRC that you are self-employed for tax purposes and register with them.
If you have ever been self-employed before in any other job you will not need to re-register as your unique taxpayer reference number (UTR) still works. Once you receive a UTR number you must update your profile.
Call HMRC on 0300 200 3504 to register as self-employed
Apply online by clicking this link www.hmrc.gov.uk