I've been sculpting with Magic Sculpt for many, many years, and I love it. Though I have recently begun using Super Sculpey for faces and occassionally torso/pelvis blanks on large figures (it's cheaper than epoxy putty for bulk rough sculpting work). I wouldn't waste your time with regular Sculpey. I've used it and never had any luck with it. I find it to bee too soft and it's rubbery when cured. Plus, when used in conjunction with epoxy putty, I've had problems because I bake off epoxy to accellerate the cure, and that causes problems with the bit done in Sculpey. It doesn't like to be reheated. However, Super Sculpey is a really fine product. I've found that likenesses come out really nice, and I can crank a decent face out in a half an hour. It's much firmer to work with and bakes to rock solid. It doesn't seem to have as much issues when put under the mild heat I use to cook off my epoxy putty either, though I have had some shrikage issues, where a crack develops at the mating point between the two materials. I know people who use Super Sculpey for everything, start to finish, but I don't like it for fine details. I only use it for faces.
For making the bodies for your figures, I'd either make the torso/pelvis blanks out of epoxy and use paper clips for the arm and leg armatures, or take 1/35th scale scrap figures, chop them down to the basic torso and pelvis, and then carve off all the detail until they are "naked", then use them for your blanks.
I'd use heads, hands and feet from commercial figures, especially if you've never sculpted before. Drill holes in the ends and stick the hands and feet on the ends of the paper clips, and also use a bit of paper clip as a spine for the head to sit on. It helps if you have a decent, well proportioned figure to compare your work with as you go. I also recomend Magic Sculpt or another comparable epoxy putty for the clothing and details. It has different properties based on how far along in the curing process it is that can be used to your advantage. There's some work I handle right after mixing, then there's other work I wait as much as 40 minutes before attempting. But first, you need to flesh out the figure, creating a basic musculature over the paper clip armature. Decide on, and set the desired pose, and then flesh the figure out with epoxy. Your figure should be taking shape now. You don't need to waste time making the arms and legs look smooth, just make sure the overall shape is the way you want it. No knobby knees, or depressions in the calves and thighs that might show up and effect the shape of the clothing. You can always go back and fill these annomalies in with fresh putty after it cures, or scrape or sand away material.
Most wrinkles are best done later in the curing process IMO, especially tight wrinkles. You'll want to spread the putty over the work area while it's still fresh, but let it sit for a while before doing anything with it. Again, practice comes in handy here, because the drawback to letting it sit is that your working time is nearly gone and you'll need to work fast. You might want to tackle small areas at a time to avoid having spots cure before you've had a chance to work them.
Sculpting wrinkles can get very teious, and making them in scale can also be difficult at times. Study life sized articles of clothing. If you have pics of the specific garment you are trying to model, study how the wrinkles behave. There's a tendancy to model thin, loose clothing as too billowy, or too thick. I'm thinking of several of Verlinden's 120mm Vietnam figures, particularly the earlier efforts. It can be easier in some respects, but also harder in small scales like 1/35th and 1/32nd. Again, practice is the key. If you have pics of some figures in which you feel the sculptor did a particularly nice job in capturing the wrinkles, reference them often. Observing the work of others is very important and you can learn alot.