Then pull any dents with a "stud welder". This will reduce the amount of body filler needed. Anywhere you will be applying a stud, grind to bare metal to create contact with the welder. Once a stud is welded to the metal you will attach the slide hammer to the stud and pull on the slide hammer while tapping on the area surrounding the dent. As Newton proved "for every action, there is an opposite but equal reaction...". So normally there will be high spots around the perimeter of the dent. As you pull on the slide hammer and tap the high spots the dent should being to "walk" out. (note: this will take some time, but with practice this procedure will become more clear)
After any large dents are pulled it is now time to apply the filler. First grind down the work area with 80 grit sand paper on a dual-action sander. Use the sander flat and you will slowly start to see the different layers of paint/primer. Any dents will become apparent because they will remain painted and the area around the dent will become bare metal.
I prefer the brand "Rage Gold" when it comes to body filler, and is trusted in most body shops. You will need a flat, non-porous, surface to mix the filler on. Everyone has their own techniques when it comes to using filler. I like to apply a good amount on my first coat. This first coat is where the majority of the "fill" should come from. Use a spreader to put down as much filler that is needed to fill the dent and overlap surrounding area. Let harden, and then begin to sand with 36 or 80 (depending on size and depth of work area) grit sandpaper on a block. Sand in a diagonal pattern // then \\ crisscrossing and creating an imaginary "X" over the work area. Do this until you level the filler with the surrounding metal. This will take some practice to get correct. A helpful trick is to apply a guide coat of black spray paint to see if there is any low spots or high spots in the filler.
Continue this process of applying filler and then smoothing, and after every coat go to a higher grade sandpaper. (80,180,220,320,400). Stop when you feel, with your hand, that the surface is level with the surrounding area and there is no bumps, or dimples. You will finish the filler with 400 grit before applying the glaze. Glaze is basically used to fill any fine scratches still left in the surrounding metal from sanding, or pin-holes that are in the filler. You will apply this extremely thin over the filler with a decent amount of pressure to fill any imperfections. Let the glaze dry and sand with 400 grit using the same process as before, but not so much that you start to take off material from the filler below. You will see the glaze fill the scratches and pin-holes.
Now sand the entire work area that you will be priming with 400 grit sand paper. "Feather out", sand any lips from the various layers of paint to make sure there are no ridges that will be visible when painting. If you can feel it with your fingers you will definitely be able to see it when itís painted.
Mask up for high build primer by back taping your work area and covering anything you do not wish to get primer on. Never apply primer to the edge of the tape or you will get a "hard edge". Apply "self etching primer" to any bare metal. Then use high build over the work area.
Let the primer dry for the recommended time, and wet sand with 400/600 grit paper. Use the same X pattern over the whole area. Be careful to not sand through the primer.
That is pretty much the basics. If anyone is in need of further information or clarification just let me know. Finished product (instead of going to the down draft I brought it to my friend's garage with a homemade booth. I did this to stress upon the do-it-yourself title, and show that a 80,000$ booth isn't necessary for a quality job, although it does make it easier):