Step 0: You'll need:
 
 two cotton tank bras. I like the racerback style, but really anything will work. Matching the bra colors is optional. I find that the multi-packs from the various resellers work pretty well and give a reasonable return.
 
 thread color to match the outer bra.
 
 a sewing machine that's capable of doing a 'zig-zag' stitch. (that's pretty much all of them except the cheesy hand-held ones.)
 
 
 
 Step 1: take two bras and put one inside the other.