|A lot of customizers begin in the 3 3/4" line, and then move on to the more demanding 12" dolls. Most of the same techniques at this scale are the same, with a few new skills thrown in.
Soft Goods - the term used to describe cloth clothing or accessories. If you want to duplicate the 12" scale dolls, you'll need to learn to sew, or have someone able to sew doll clothing for you. This will involve making patterns, choosing lightweight material, and experimenting with velcro, snaps and minature buttons or clips. You may be able to purchase sewing patterns for generic items such as pants and jackets -- check out the pattern section of your local fabric store for "doll" or "toy" specific patterns (ie. Barbie or GI Joe patterns). Alternatively, remove the clothes from your existing 12" dolls and turn them inside-out ... you may be able to make your own patterns by following the shapes from these professionally-made items. When choosing fabric, make sure it's lightweight so it will hang properly, and that any pattern on the fabric is of very small dimensions to keep in scale with the doll. Check out the bargain bin of roll-ends for good deals! If you're doing a furry alien, fake fur can be found there as well.
Parts - If you can't bear to alter your Star Wars dolls, be on the lookout for cheaper dolls with good articulation and natural facial features (not comic, or anime styled). Many customizers use GI Joe dolls, but these can be pricey. Female bodies can be made from Barbies, but the style isn't exactly the same as the Star Wars line, and articulation is limited in most of these dolls. If you're doing an alien anyway, head matching is not a factor....you can scratch sculpt a head on a well-articulated body (ie. Hammerhead, etc.).
Accessories - You can purchase clothing or accessories such as shoes, boots and belts that fit other 12" dolls. Or, you may need to make your own. Shoes and boots would be extremely difficult to make, but hats, belts, pouches and the like would be easier and cheaper to make from scratch. Again, the fabric store has a wide variety of "fake leather" fabric, plastics, netting and "fake furs" to choose from.