First, there are several choices in the product you use. Bondo brand filler is terrible to work with, but is cheap. I would recommend spending a little more (about 25-35 dollars) for a gallon of a premium lightweight filler. Evercoat makes several different fillers, i would recomend their rage line. It spreads much nicer than bondo brand and gets less pinholes, which will dramatically reduce your sanding time. As for the actual application, first prep the area that you will be applying the filler to with a minimum of 80 grit, you could even use 50 or 36 grit. This coarse scratch will ensure that you get a strong adhesion of the bondo to your panel so it doesn't chip off down the road. The best time to actually begin sanding is just as the filler (bondo) is hardening, usually just a few minutes after applying it, depending on how much hardener you used. You don't want it to be clogging up the paper, but it is much easier to work with when fresh. Use 36 grit at first to shape the area, then 80, 150, 280. Once you have the shape how you want it and it is smooth, its ready to prime. Hope this helps a little, if you have any more specific questions just send me a message and i would be more than happy to give you some pointers.

Eric