At the end of every financial year you need to complete a tax return. This can be done online via the HMRC website here.
First you'll need to register as self-employed. Before you can submit a tax return, you’ll need to make sure you’ve registered as self-employed with HMRC. You can follow the steps in this guide to register.
Next you'll submit your tax return. Once registered, you can log in to your HMRC account and complete your tax return online. You’ll find helpful guidance and step-by-step instructions on the HMRC site as you go.
Need help? HMRC offers guidance and support for anyone submitting their return for the first time.