go to WalMart, (in automotive) get a fiberglass repair kit. Or get fiberglass mat, resin, etc. and follow directions. It's pretty messy and you may want to learn how to use the stuff on something else first...
First of all, the fiberlass mat(the uglier looking stuff) is stronger than the cloth, because the individual fiberglass threads are bonding in more directions. First you take the bumper off, and from the back of the bumper you re-align the cracks if needed. Mix your fiberglass resin with the liquid hardner with the amounts is says on the can. Then with a very short paintbrush (bristles cut to about 3/4" to 1", paint some of the mixture onto the bare fiberglass. then take your fiberglass mat, and lay it on top. press it with your fingers (use latex gloves. Dont rub too much, or the mat willl come apart and create a mess.) Make sure that the mat becomes completely saturated with the mixture from underneath. If needed, use the brush to apply some extra mixture to parts that are dry. after this, you would probably want to put another layer or two on, just to make sure it won't go anywhere. it all depends on how it feels to you. but don't forget to let the cloth dry inbetween each coat. I hope this gets you starrted.
well i mean bondo (putty) is really meant to fix the outside of a body panel like a dent or a deep scratch (for lazy people who don't want to go to a real shop to have the panel re-rolled and smoothened with tools) . it would be benificial to put a layer of that stuff over the outside of the crack to make the final result smoother (which you would obviously have to sand and re-paint) but honestly, what holds fiberglass together better than fiberglass. to be honest, i'm not even sure if bondo (putty) has strong enough adhesive properties to do the job, its mostly just a filler. it depends though, how big is the crack?
anything more than 2-3 inches, and fiberglass will be the only way to go, otherwise you may be able to get by with the putty (not guarenteed to hold more than a few months, or after a tap to the curb tho).