Its easy, after you have used body filler and sanded that down a decent amount, there are normally still small holes that bondo might not quite get in, for these you use spot putty. Dont use it on large holes, just small ones, like pen tip sized holes, any parger than that and i would suggest more body filler.
Easy to apply, no need to mix like bondo, just squeeze it out of the tube (only a tiny bit at a time) and dab it on the spot that needs filling, wait till its dry, sand, done!