How to wet sand

Wet sanding involves using a specific type of sandpaper. The benefits are less loading of material in the paper, a better visual view of the sanded area (the sanding dust is rinsed away), and allows the use of a much smaller grade of sandpaper than dry sanding (anything over 400 grit tends to load the paper almost immediately).
The drawbacks are that is is a bit messy, and that the surface has to be thoroughly dried before painting, as water can become trapped in seams and moldings. Wet sanding isn't very complex. Water, some 600 grit wet paper and a lot of elbow grease.
Sand until you don't see any orange peel (it shows up well when the water dries) and small imperfections are feathered. Use the block to squeegee off the excess water. If the water you are using has a high mineral content, don't let it dry on the car. It could leave spots which may bleed through the new paint.