It's fairly easy.
I never did it before either, but I just bought the stuff and did it.
It's pretty straight-forward, rip the fiberglass mats to pieces you think are easy to work with (I'd say about 15x20 cm), mix the resin and the hardener, place a piece of fiberglass and start putting on the resin with a brush with a 'tipping' movement, don't stroke.
Keep doing this until it has the thickness you want.
I found this link
very usefull, it seems offline for now however.
Just make sure you got a well-ventilated space!
The fumes are... well after a few minutes they are pretty nice actually, and so are the colors and those pink elephants.. be carefull with the fumes.