Well, there is definitely pros and cons to both mold less construction and making molds. I have done both fairly extensively, and as a rule do not make molds unless I am either very sure that I want to make a duplicate, or the fabrication is so complex that I want to never have to do it again even if wanting another part is not especially likely. That said, unless you want to make more than one part, there is really no value at all in making a mold. Making a perfect part to cast a mold from is no more work than simply making the part itself, and then you also have to make the mold and then the part!
There are plenty of how to's on the interweb, and the whole process is really very easy, forgiving, and straight forward. For most people the first hurdle to overcome is just getting used to using the materials. Unless you are making airplane parts, you can be pretty fearless as you can cut and modify anything you create pretty much infinitely without having too much concern over strength and functionality. Make something a bit too small or accidentally cut off too much and you can simply add some more glass and all is well. I use epoxy instead of polyester resin as it is much stronger and generally has much better working time, but it is considerably more expensive. If you are needing light and strong parts, epoxy is the way to go, but for most bike projects, polyester is fine.
Most beginners greatly underestimate the need for careful and thorough planing. It is nearly impossible to make mistakes you can't recover from, but you can create a monstrous amount of needless extra work for yourself. If you are going to carve up some foam into the shape you want and glass over that to create your part, don't imagine that you can just make it close and then fine tune the glass. Make your foam shape absolutely perfect first! Glass over a really carefully made form and the work needed to get it ready to paint will be very minimal.
Once you are ready to start laying up the glass, get very prepared. The resin starts to set up more rapidly than most people expect at first, and they also usually under estimate how long it takes to cut and saturate the glass - and how crazy messy it can get if you are not organized. Carefully pre-cut all the pieces you intend to apply and lay them out in order so you can place them quickly and easily. The biggest thing for beginners to overcome in actually doing the layups is the fact that both polyester and epoxy resins have very nearly zero adhesive characteristics. This makes it extremely difficult to make the layers conform to anything even remotely resembling a corner. One way that is good for the inexperienced to overcome this problem is to paint your form with a thin coat of resin first. Then wait for it to start to kick, and them just stick your first layup to the now very sticky resin. With a little determination you will be able to get your layup to stick smoothly and tightly down to even a difficult shape. You do not want to saturate your cloth at this time, you only want enough resin to glue the underside of the cloth to your form. Once it is stuck down tight, you can come back with a new batch of resin and saturate the cloth. You can repeat this process to build up as many layers as you like. If you are careful and think ahead, you can end up with a shape that needs only minimal work to perfect for painting.
Also, figure out how the rest of your project is going to be made. I think in most cases, you get a better end result with a separate seat pan that sits on top of your seat base and tail section. Once you make the base, it is super easy to make a seat pan for the upholstered section by using the base as a mold for the pan. And don't plan on stapling your upholstery to the fiberglass pan. Staples and fiberglass do not get along at all well with each other!
