Its not veary hard. you take the foam and get the shape you want then you take bondo (Fumed celicut, micro bubbles, resin, and hardner. THis stuff works better then bondo off the shelf you can mix it how you want in.) when you get it all smooth you take a mold. and then make a part (This is the hard part). The key is to take your time. sounds really hard but its not to hard just start with small pieces. I would start out with stuff like this just to get good with fiberglass. :thumbup
the foam will melt when the resin cures, making the foam surface all wavy. thats why you use bondo on top. i know since i wasn't too careful and overshot while making my lip (which i have no time to finish at the moment).
the foam+bondo (or plaster, or anything else heat and chemically resistant) are a "plug" or "male" mold, the fiberglass piece pulled from that will be the "master" or "production" or "female" mold which will be used to make parts identical to the shape of the foam plug.
plug in early stages, foam blocking
later on with bondo and fiberglass added.
What I'm doing is actually skipping the plug>mold>part process by making a foam shape, and then taking the foam out from the inside leaving only the bondo and 1 layer of fiberglass cloth i put on for strength. it's a VERY tricky process. once the foam is gone, fiberglass is layer in from the inside to add strength. it's sometimes used for one off parts or to test a part out before spending money and time on molds.
I was stupid enough to remove alot of the bondo on the lip part, so when i took out the foam, the lip collapsed. I've had to go back and put tape to hold to upper and lower part of the lip while i layer in the fiberglass. it's not a pretty process. actually the whole thing is a very messy process but if you keep working on it and put effort into it, you can make anthing you want for your car...or boat..or whatever :thumbup
try it out, the lip was my first venture into serious fiberglassing.
also USE GELCOAT WHEN MAKING MOLDS, it will save you so much time later on given that the plug is sanded to a smooth finish. a rough mold will mean body work on every panel made.
your first mold should need some work. dont try to get it perfect the first time. its easy to take to much off in the foam stage. but hard in the FG stage so its a little hard to get it the first time. but with practice you can get it.