I could write a small book on this topic. My screen name comes mostly from my composites work actually.

Ok, so where to start. I'm going to keep this somewhat general, but focus on what would work well for something like the sideskirts you want.
I'm writing this off the top of my head so the organization will suck, but that's OK.

First, some background and clarification on what's already been said.

A quick note on "special resins": The "special resins" that fiberglass companies use aren't made by them, they're made for them. Polymer chemistry isn't something you can just play with, some shop can't mix a bunch of resins together and come up with some new resin that works. Doesn't happen. It's possible to call up a resin supplier or manufacturer and get a resin tailored to the properties that you want, such as pot life (time before gel) exotherm, flexibility, toughness, etc. There are always tradeoffs obviously, and you can't have the best of all worlds for what you're doing. Most custom resins require huge min quantities, although there are a lot of really cool resins already on the market (like flex resin) that are available if you want to buy a drum of it, or at least 5 gallons for commonly available stuff.

Fiberglass: There are a lot of types of fiberglass on the market, all different based on weight (grams/m^2, or oz/yard, basically weight per area), weave type, and fiberglass type (don't worry about that part too much). So when someone says "make it out of fiberglass", that's not really saying anything. Briefly, heavier weights are generally thicker and will build up thickness per layer faster. There are plain (basket) weaves with big wide fiber bundles (called roving) that are very thick, fine weave fabrics with very good strength, and random fiber mats (chopped strand mat). The chopped strand mat is the cheapest, easiest to work with, and has the worst physical properties of anything you can buy. There are also specialty fabrics like biaxial and triaxial, which are just layers of (non-woven) bundles which alternate directions in each layer, kind of like the wood grain in plywood. There are many others, and many subtypes of the ones I've already listed, but I'll let you see what's out there.

Resins:
Polyester: The common "fiberglass resin" is polyester resin. This is the stuff that smells really bad (actually, I love that smell). It's fairly cheap, there are lots of kinds available, and special kinds for special uses. General purpose layup resins are usually garbage, so look for a premium layup resin at the bare minimum. The price difference is only a few bucks a gallon, if that. Never buy resin from Lowes or any place like that, it's crap and way overpriced. Shrinkage of up to a couple percent by volume. That could mean length or width or both, depending on the shape of the part. The amount of reinforcement (fiber to resin ratio), the cross sectional area (thickness of curing resin) and the cure time all affect the amount of shrinkage.

Vinylester: In between polyester and epoxy for price and performance. Not as easy to find as either, and I don't usually use it because I'm either going for cheap or strong, not in between. Chemically similar to polyester though, with a similar smell. Shrinkage not as bad as polyester.

Epoxy: Ah, the good stuff. Most expensive (and there is a HUGE price range) and the best physical properties. Mix ratio is critical usually, or the resin will never properly cure. There are some epoxy systems which are more forgiving than others, and some which can be physically tailored through different mix ratios (those are rare). So plan on a very accurate measuring or weighing system. Most epoxies don't like UV light, so they have to be painted, and epoxies usually don't take well to sanding (and especially not polishing). Shrinkage is negligible, and usually too small to measure.

Generally, for room temp cure composites (no ovens, post curing, etc.) you can save money and make molds out of polyester resin. It's easier to maintain them because of the ease of sanding and polishing, and polyester gel coat systems are a fraction of the price of epoxy gel coat systems. You'll use a lot of resin for the molds, so low price is good.
Gel coat is usually used as the outer surface of a mold, because it is easy to maintain and polish without exposing raw fibers, and it gives some thickness to prevent print-through. Oh, print through.. Print through is what happens when polyester resin shrinks during the cure, conforming to the fibers or fabric weave and leaving the weave texture visible through the surface of the part. I'm sure you've seen cheap fiberglass products that this is visible on. There are various things like surfacing veil that can be used to help that, but that's a mini-novel in itself.

Ok, so on to molds. Molds are a huge topic, and I'm going to make it short. Generally, molds are a lot of layers of fiberglass, and need to be sturdy. You don't want a mold's surface to flex while you're laying up a part in it, and you don't want it to crack while you're pulling a cured part out. So molds are usually pretty damn beefy. They're also usually braced with some type of stiffeners or structures to keep the whole mold from twisting or distorting depending on what it's resting on, etc. The easiest way to make a crooked part is to make it in a crooked mold..
Molds are usually made on a finished or semi-finished part (you can buff the mold before using needed) starting with gel coat against the part surface. Mold release goes first obviously, so it will actually come off. Gel coats are usually sprayed, but some can be brushed with OK results. After the gel coat, surfacing veil and a few layers of chopped strand mat go down to prevent print through. Once this is built up enough, the big reinforcements can go down, like woving roving or heavy cloth to build up stiffness and strength. Sometimes bulkers like coremat are used for the same purpose (cheap, fast buildup) with another layer or two of fiberglass over the top of that. Next the stiffeners go on, if necessary. MDF cut to shape works well, with intermittant pieces of fiberglass to secure the MDF to the mold.
All of those steps are done with curing in between, so there aren't big thickness curing at the same time which would lead to lots of shrinking (read: warping) and a lot of exotherm. Good molds take a loooong time, make no mistake. Don't forget buffing so your parts come out nice and easy.

That is the same process that any shop would do when copying an existing part. If you already have a part to make a mold from, it's that much easier.

Okay, so you want to make a part. You've put a lot of layers of mold release in your mold, so you part comes out. You can choose whether or not to use gel coat, and you can often skip that step if you won't need to do any sanding and the part will get painted. Your fabric choice will depend on what properties you want, strength, cost, etc, and you can research the tradeoffs on your own. So you put down a layer of fabric, apply resin with a brush or roller (there are special resin-proof paint rollers), put down another layer or two, and use a squeegee or ribbed roller to compact the fiberglass. When you have enough layers, you obviously let it cure. How many layers depends, again, on your choice of properties. Fiberglass is cheap, so a test piece isn't a bad idea. Whatever you're doing, it really helps to practice beforehand so you get a feel for what you're doing and how everything interacts.

So back a few steps.. You want to make a foam plug (pattern) for a side skirt. You have a few types of foam to work with, each (again) with its own properties. Rigid urethane foam (usually polyurethane) and isocyanate foam, each with many different types (polyisocyanate, etc) and different available densities, are really good to work with but not necessarily cheap. Polystyrene foam, which is what house insulating foam is, works OK and isn't nearly as nice to work with. It's attacked by polyester resin, because the solvent is pretty crazy (thats what gives it the smell). It's really cheap though. If you want to use polyester resin (and why not) you can cover the foam with epoxy and take the mold off of that, or make the mold with epoxy ($$$), or cover the foam with something else. If the part is simple, good quality clear packing take works really well usually. If you make the tape job look good, all you'll have to do is cover the tape with mold release before you make your mold.

Oh, and "flex vs brittle" mostly has to do with the type of resin, the part thickness, type of reinforcement, and the fiber to resin ratio. The resin itself is only part of the puzzle.



Ok I'm done for now, back to my homework.

Oh, and fuck Cliffs Notes, you can't learn anything that way. Read the whole post.
I typed it all out, you can read it all.