personally i'd get a bodyshop to do it. if you have no experience with it you will most likely make a mistake and it probably won't come out looking smooth. if you want, pay attention to someone else while they do it and that way you will have gained some first-hand knowledge for the next time.
cracking mainly depends on where you mold. obviously if you are in a wreck anything can crack. that said..some places are more prone to cracking than others. fender flares should be bolted on and then molded. this reduces the potential to crack.
sideskirts are also a nice place to mold because they are relatively untouched by the road.
front bumpers shouldnt be molded unless you are making a show-only car. your front bumper will amost always hit something (be it a curb, speed bump, rock, etc.) and this will cause it to crack.
rear bumpers are almost the same as front bumpers--not unless it's a show car...rear bumpers have a high potential to be wrecked (just like a front bumper) and it can also be more prone to skidding & dragging, which will crack molding