How to create a brand new general mod for GTA San Andreas ? Thread

Greetings,

Sorry for this beginner question, but I haven't found any answer.

I was wondering how you can create a brand new general mod for GTA San Andreas; like, not only adding missions, but removing and replacing all the base missions with custom ones, removing all the peds and cars from the game, specific interactions, etc.

I thought it would be DYOM, but apparently not.. or am I wrong ?

Thanks.