What is the best way to paint latex paint over enamel paint ?

Enamel can be latex or oil based. Do you mean a very smooth, shiny, oil paint? Is it interior or exterior, wood or metal?

If it is interior solvent based enamel, you should be able to sand the enamel with 220 or 320 grit and repaint. If Latex enamel, it is harder to sand but still needs to be done to give the enamel a "tooth". I would also use a good primer over the enamel.