Totally depends on how good you want it to look & how much time you're willing to spend.....
If you want it perfect, it's just like painting anything else.... sand, sand, sand, sand, sand.... wash it down, get in the right environment (painting booth), spray multiple light coats... wetsand, spray, wetsand, spray, wetsand, spray... clear, clear, clear, clear, clear....
get the picture?
If you just wanna do a quicky job, sand the existing paint w/ very fine sandpaper, wash it down & spray right over it.