A. You can either use very fine wet sandpaper or very fine disks that attach to your DA sander. I now use mostly disks because it makes the job go a lot faster but if I need to take out a nib or run I use a Nib File and/or wet sandpaper on a Run Blocker in order to take down the high spots. For orange peel and just making the surface flat I use 3M Finishing Film disks.
B. If you're wet sanding be sure to keep the sanding area flushed with water. Allowing the surface to drain off too much will cause the paper to wrinkle or allow particles to remain on the surface where they can do damage. I don't like block sanding for most of my wet sanding unless I'm working on a run or a nib. Fold the paper in thirds and grip the open end between your thumb and index finger and sand at an angle so that your fingers don't cut groves in the finish or wrap the paper around a soft sponge-like block. It best to sand in straight lines not in circles because circles can wrinkle the paper causing scratches in the surface. Sand in straight lines but vary the location and/or angle so that you're not continually allowing the paper to track in exactly the same place ( tracking along a molding ) more than two or three strokes. During the wet sanding process you can use a soft rubber squeegee to wipe the water from the surface so that you can see if you've sanded enough or need to do more.
C. If you are power sanding don't sand right up to the edge of a panel. Stay back about and inch so that you don't need to apply so much buffer action right up to the edge. It's buffing hard on the edges and peaks that will quickly wipe the paint from these surfaces. Using Finishing Film will produce a powder that will act as a guide coat and allow you to see your progress clearly.
A spinning buffer will tend to draw loose clothing into it, be sure to wear coveralls or keep shirts etc. away from the action of the pad. Do not clean the spinning pad with anything other that the proper tools, screwdrivers and other tools can become very dangerous when used improperly. Because buffing tends to increase the amount of material in the air eye and breathing protection are a must.
D. One of the reasons I recommend Sure Finish or Presta Ultra Cutting Cream is that these products cut more effectively as as you buff you may spray the adjacent panels with compound, if the material is more water based it will dry quickly and you'll cause deep swirl scratches in the panels as you begin buffing on the panels that are covered with this overspray. With the Ultra Cutting Cream you don't get this condition as easily unless you leave the material to dry for a much longer time and the Sure Finish flings very little product onto adjacent panels when used as directed.
E. While buffing be sure that the buffer rotates OFF of the edges and peaks. If you allow the rotation of the pad to beat against the edge by rotating ONTO the edge it can instantly remove the paint from the edges and peaks.
F. I use the Presta from the pail and apply it with a 2" paint brush that I leave in the pail when not in use. Take the brush and apply a 4 to 6" smear of material on the surface then, without triggering the buffer spread the compound around with the pad then feather the trigger while moving the buffer back and forth to distribute the material without throwing it off. After the material has been spread pull the trigger but keep the rotation of your buffer set at or below 1000 RPM for compounding. When the compound is gone keep buffing until you feel that the cutting process is finished then stop and turn the buffer over and SPUR the pad to clean off the old compound before you apply any more compound. When using Sure Finish I use it differently by dampening the pad then conditioning the pad prior to compounding, so follow the instructions when using Sure Finish and your results will be outstanding.
G. If you're buffing a surface that has been sanded you will need to repeat the buffing step above twice to be sure that you've cut enough to remove all the scratches. The oil from the compound will hide the small scratches and unless you cut the surface enough the scratches will appear once the oils have been removed and the true nature of the finish can be seen.