Depending on what you are actually painting will dictate how fine of grit sand paper you will use. If the surface will be seen and has to have good color holdout, then step you paper up to the 320-360 range.
Step one: thoroughly clean the surface with a prep-solve type of cleaner. Do not use lacquer thinner as it will leave a film on the surface. A synthetic not acrylic enamel reducer will work. For reference Dupont 3812.
Sand and re-clean with prep-solve
If possible, I use a wet on wet method.
Spray two light coats of etching primer
After ample flash time, apply your topcoat. This wet on wet application will have far superior adhesion properties vs sanding between primer and paint and better color holdout