If you have no runs, just wetsand it until it's smooth (you can see this by wiping the area with water or mineral spirits), which normally doesn't take too much paint off. But it removes the gloss, and there's a fresh cut potato smell (!). I had some runs in my most recent test area, and I'm having to wetsand more because I do the whole area, not just where the run is. In that case, sand until the run disappears.

Also when you're wetsanding, when you spray the surface it will be tiny water drops. After sanding, if you spray water, the water will be more evenly dispersed. It's a good way to tell if you've missed an area with your sandpaper. Also I would use a foam sanding block with the 600/800/1500 grit wrapped around it. I have been using that, and it doesn't load up like it might with a hard block.