Hi there, just lucky I saw this thread before heading off to bed.
When I perform bumper repairs - I paint the whole bumper (some smart repairers just repair & paint the affected corner, but I think that can give colour/finish matching probs).
As with all of these jobs, preparation is everything.
All non-painted trim will need to be covered with masking tape, or if possible - carefully removed.
Naturally, the areas of the immediate bodywork will also need covering.
Using a mini sander (starting with approx 180 grit sandpaper), sand (with only a light pressure) on the affected area, work this up to 320 grit, spreading over an area of increasing size. It should be smooth, with no trace of the original damage either visually or by touch. Filler is very rarely needed, and in my opinion just adds to the repair time without adding to the result.
The remainder of the bumper needs to be lightly keyed with 320 grit.
Lightly, but comprehensively dust the bumper with 2 coats of primer, and allow to dry for approx 1/2 hour between coats. Lightly rub down with 1200 grit between coats. Then with an aerosol of the correct colour, coat the bumper in the same way. (including the rubbing down).
Chances are the finish will be lacquered, apply 2 coats (be careful to get enough on for a 'shine', but avoiding runs - this can be achieved by keeping the can in motion, but no closer than 6 inches from the surface. You will need a heat source (hairdryer will do if a heat gun isn't available), to get the lacquer to glaze over rather than absorb moisture (which dulls the finish).
Allow the surfaces to fully dry before slowly removing the masking tape.
I would always advise to do this under cover, and you should allow half a day from start to finsh. I can do a bumper in 3 hours, but that's with practice, and having the best tools to allow this.
Best of luck with the repair, and if I can advise further, please don't hesitate to fire away.