how the hell do i prep for paint??

im preping a white fender for paint to go on my tri coat red mustang the frnder has one small ding (not creased) and a few smaller paint chips where i cans ee bare metal

the guy at the paint supply place said to rough it up with scotch pas till dull
1 how dull does it need to be
2 what do i do about the small paint chips currently there if im only getting off clear coat?
3 is there a better way to do this its my first time ever doing paint on a car!