Declaring historic loses to HMRC

Hello

Apologies if this has been covered before, I didn’t see it

I have loses from 2018 and 2019 that I never declared to HMRC

I’m aware you have 4 years to declare these, so I’m wanting to declare them to offset my 2021 gains

I’m unsure how to do this though, the HMRC website says if you have never done a self assessment( I haven’t) to write to them to declare the loses

Do I just write telling them I lost x amount in 2018 and they accept it? Would I need to include a full transaction report or only if they ask for it?

Alternatively when I do a self assessment for 2021 do I include the loses on there?

Thanks