Step 1: Acquire Scrap Leather
Fake fur makes an especially tasteful accent; be creative.
Step 2: Make a Bikini: Pattern
Trace the pattern onto the leather, making sure you mark the orientation properly. Add fake fur to the top, as this will (partially) help cover up the fact you've used a crappy pattern.
Step 3: Measure Twice, Cut and Sew
Trace and cut out a pattern, double-check it, then actually cut your leather and sew the thing together.
Check again, just to make sure, as this is the most squirrelly piece to fit.
Step 4: Add More Leather Pieces
Things to think about: how long/wide do I want this thing to be? Which piece of leather should I place against my skin when I overlap edges? How much overlap do these pieces need? What shapes do I want to cut? Do I need reinforcing stitches at any of the overlap points? Do I really want to put pink, red, and burgundy leather on the same top? Have I left good, strong places to place my grommets? Is this going to make me look like I'm fluent in Klingon?
Step 5: Grommet Grommet
Mark your grommet sites on the front and back pieces of your top, then get to work.
First, use the punch tool to make a grommet-sized hole in your leather. Next, pop the grommet bottom (the larger piece with a long neck) under your leather with the neck sticking up through the hole. Place the grommet top (small ring) over the neck, sandwiching the leather piece, concave side down. Place the reinforcing base underneath the whole mess, making sure the grommet bottom is fitted smoothly into the matching groove in the base. Next, take the setting tool (solid metal pencil-looking thing) and insert the tip into the top of the assembled grommet. Place the assembly on top of a piece of wood to dampen vibrations.
Hammer until the upper side of the grommet has folded over, pinning the top tightly against the leather. The top should not spin or wiggle; if it does, hammer more. The amount of smashing required will depend on the thickness of the leather you're trying to pinch inside the grommet.
Step 6: Tie It on and Go Out Dancing
Finally, you'll need to find a cute boy to lace you into this construction and keep you company while out dancing. Warn them in advance that this may take a while, and that they have to dress in complementary fashion. My boy looks pretty hot in his leather pants and shiny silver shirt (see silver shirt instructable); your mileage may vary.