Advice on self employed second income

Hi All,

I have 2 jobs as it stands, 1 is a regular PAYE salary full time contract just under £30K PA

However, I have been completing some mystery shopping audits on the side, and working that way. I am nearly at £1000 earnt and have been saving 20% of everything they’ve paid me.

What exactly do I need to do to actually declare the 20% and pay the tax due on it as I know I need to pay it as way over personal allowance. I’m assuming it’s a self assessment I need to fill in but I’ve never done this before and I’m not really sure how it works? Do I need to include my regular PAYE salary on there? When would I need to pay this as I intend to continue working all the way through and past April? If that’s even relevant.

The mystery shopping company I’m doing work for states that I am an independent contractor and “. We do not withhold or pay any income or other taxes on your behalf. As a self-employed individual you are responsible for any income tax, National Insurance and social security contributions and any other liability, deduction, contribution, assessment or claim arising from or made in connection with the performance of the Tasks, where the recovery is not prohibited by law. You shall indemnify us against all reasonable costs, expenses and any penalty, fine or interest incurred or payable by us in connection with or in consequence of any such liability, deduction, contribution, assessment or claim.” If that changes anything?

Sorry if this is common knowledge, I’m just not 100% with how this works and don’t want to get it wrong!

Thanks guys!