I prefer Bondo to Spackle. It shrinks less, cures faster, and sticks better. For small holes, like screw holes, you can just force it into the hole. Larger holes need to be backed up somehow. I generally slip a small piece of wood into the hole that is as wide as the hole's largest dimension and 2x longer than the width. I tie a sting to the middle of the piece of wood, spread glue onto the wood, then slip it into the hole. I then use the string to pull the wood up against the inside of the wall and tie it to something to hold it in place until the glue dries. Once dried, I hit the crater with Bondo. If more than one or two inches in diameter, I also like to embed some drywall mesh in the Bondo to ensure it will not crack.