Well it all depends. If you get a run sand with 600 to level it off. If you get dust or dirt or anything use 600 to get them suckers out. When your happy with as many coats as u want, start with 600then go to 800,1000,1200,1600,2000. Sand with the 600 till all the shiny spots are gone. Then just keep sanding up the line until the previous scratch marks are out. Around 1200 grit youll notice the guitar starting to reflect things. When your done with 2000 go here for polishing it up. Make sure to use a sanding block because your fingers can acualy put little "furrows" into the paint.
http://projectguitar.ibforums.com/index.php?showtopic=1723
Make sure to always wet sand! and check you passes to be careful you dont burn through the clear. It really sucks when that happens.
Also be careful on the edges, theres always less lacquer there and its easier to sand through. Hope i answered any questions