Yep, it is easier said than done sometimes and it depends on the design and what you want to achieve.
I would do a search here on this site or on Google and read away. While you can use fleece it also gives a rougher outside surface that requires more filler or sanding to get smooth but if it's being covered with padding and material it is a way to get a quick first layer on top of a mold or from being streched over something. Normally the stuff I've done I've used double knit material (a little thinner than T-shirt material) but one layer of it is fairly thin and requires backup matting and fiberglass to make it stronger. On some of the sub enclosures I've done I've also used speaker box carpeting but you have to watch how thick the layers are as the process is a chemical reaction and creates a fair amount of heat which can also warp panels on the project.
Jim