Freelance Tax Questions

Hello all,

I have been asked to do some graphic design work on a freelance basis for my current employer, who employ me full time in a different role.

It is expected that they will need a project every other month to start, so i think i need to let HMRC know. What do i need to do for tax? is this just 'other income' in self assessment? or do i need to do more?

Thanks, J