All you need is a color printer and some self-adhesive paper (like the kind you make for address labels, or Crack-n-Peel sticky paper found in office supply and craft stores).
Simply save the images below by right-clicking on your mouse, save to your harddrive and then print out on sticky paper. Then cut your stickers for the desired shape and start sticking them on anything you want!
Check out these pics of places fans have shown off their DIY stickers.
Or if you don't have a printer at home, you can make these stickers at Kinkos or another copy place. Just download one of these designs, ask for sticker paper (like Crack-n-Peel) and then copy it to that paper. Each copy costs less than $1 and depending on the size of the image, you can get around 15 stickers a page.
If you want fancier stickers that are usually used for sticker bombing or for decorating cool stuff like skate decks, guitars and more -- you can order 250 vinyl stickers for $20 at the StickerGuy.com!
More cool Star Wars stickers can be found here:
Vader is Back Stickers: