Always repaired from behind, no the front, if you want to do it properly, grind the backside until it is thin on the edges and all cracked and busted old glass is gone, you do t have to remove all, but an old crack will bite you later, rough up surrounding areas with grinder and a sanding disk on it. Always glass to a prepped surface.
Now realign bumper and hold In place with packaging tape, wider the better, as tight and as straight as you can, you can glass straight to the tape, when its dry you can pull the tape right off.
Sand outside surface, then bog it back to shape, etch prime, build coat with a sandable primer, make sure at this point something around the 240 wet and dry to remove all deep scratches, top coat.
Mixing resien follow instructions to the tea, use a brush, make sure you push all air out, start out in small amounts until you get use to it, open space is a must,
And last thing is when icthy, have a really hot shower until it stops stinging, it\'s the only way to get the fibres out of your pores, also have a go it is fun and easy once you get going. And if you get into trouble give me a yell, will happily help out where I can .