Get some fiberglass mat, resin, hardener, brushes and some gloves and just mess with it.
You can't start with over fenders, well, you can but you will give up.
Don't worry about mould making, expanding foam or ANYTHING yet.
I would start by doing this; Get something simply shaped and tape it up with painters tape, spray some Pam cooking spray on it (This will help with mold release). Mix your resin & hardener. Dip your brush into the cup, remember FG eats through things so you will go through cups/gloves/brushes relatively quickly.
Get a layer of your mixed resin on the piece you want to copy. You should have cut up your FG mat before you started. Start laying your mat on the piece and dab the mat with your brush. Once it starts to stick, you can get it a bit more wet. Just keep doing this until you have no un-fiberglassed areas. Try to avoid air bubbles and stretching out your mat too much. (Air bubbles won't kill you but its going to be a PITA when you go to do your finish work)
Let it settle for a few min (You will have to learn this with experience as to what the proper time is) and add another layer. (If it is to thin, as you go to pull it off it will break in half)
Once you get proper coverage on the piece, let it dry. Once you think its dry, let it dry some more. (Once again, you have to do this a few times to learn what is right and what is not) It will be kind of sticky for a while, I like to pull my copy off when its no longer sticky.
Break your new piece off and clean it up (dremel etc) / add more layers or whatever.
Sorry for the horrible explanation but this is the most basic way to learn. Once you get the jist of how the glass works and get your style of laying it down just right you can start tackling real projects.
Here are some sites to help:
If this is something more people are interested in I will do a write-up for you guys. One that is somewhat readable