Don't.

I once advised someone to "just be honest with your new employer as they sound like they're thrilled to have you and would appreciate your forthrightness".

I was freaking wrong and they fired the dude.

Employers have nothing to gain and will just take the next candidate rather than wasting time and money training you only for you to move to part time or quit.

They might try and accommodate someone who is a great worker, so you might try the overachiever route but no guarantees.

If they get mad about it and won't with you, then move on and find something else.