Pretty much the same as with filler. I sand the metal with 80 grit (tooth for mechanical adhesion), blow it off, wipe it down with a cleaner/solvent (I use lacquer thinner on bare metal) and hit the area with a tack rag. Mix the filler up with the hardener, and apply with a Bondo spreader. It does take a bit longer to harden than regular filler, so don't panic if it isn't hard in 3-4 minutes. Mix it carefully. When they say 15 drops to a golf ball size amount of filler, they mean it.
