how do i prepare a painted bad drywall job so i can repair and paint?

Whenever I put up drywall or fix an old wall, first, I paint it with 'scrap' paint, then sand it either by hand or with a palm sander. Then I put a paper thin coat of drywall mud on the entire wall. Then lightly sand, then it's much better. If the wall were to be real bad, just do it twice.

you have to sand it out. Chose a medium grade sandpaper - prepare for a freaking mess....and go for it.

Sand lightly in a circular motion - gently sanding down the bumpy parts. Be patient. Sand softly and don't press too hard or you will end up with gouges where you will notice a mess when you paint.

Once you are done sanding, wipe with a slightly damp cloth then prime. Once the primer is dry - paint!