First you'll have to determine if the door metal is sound enough for bondo. If it oil cans a lot, you'll run into problems sanding it flat, and the bondo and paint may crack later on due to flexing. It should not indent much under sanding pressure.
Depending on how much work you want to do and how comfortable you are, you may look into new door skins. I recommend factory if you go this route.
The longer the board you use the straighter your work will be. IMO a long board from your local auto body supply store is a must. It's about 2 ft long, and has waffled sanding surface on the bottom that helps it cut and not load up the paper as easy (make sure you get a flat one). Get some "guide coat" so you can see the hi-low spots easy. The best tip in getting it straight is to get at least 90% of the shape done in 36 grit, and use fresh paper so it cuts the shape fast and even. Work your way from an edge that you know is straight. Fill in low spots with bondo. This is a fine line here, but remember the more you put on, the more you sand off, and at the same time, if you don't put enough on and put more on later, blending is more difficult as different aged bondo sands differently. Just right takes some getting used to.
You can move to 80 grit when you are convinced the 36 grit has done it's job. If you try and shape with 80 grit, you won't get it as straight since the finer grit won't cut into the waves, instead it rides them. When the guidecoat sands off uniformly, go over the worst imperfections with finishing bondo.
Next, sanding primer will really let you know where you're at. Spray the whold door. Use guide coat and 100-120 to break it, then move to 220-320 and you'll see very well overall where you are. Finish with 400 (500 wet/dry if you are painting it a dark color). Wipe it down with grease remover, and site it when it's still wet; it will look this way with paint on it if you do it right.
Sand in an "x" pattern using long even strokes where you can.