not undies at all, to be honest. bikini top! it's pretty easy, hopefully the pictures are demonstrative enough.
the straps are a bitch to turn inside out, but worth it (and arguably necessary). for another bikini top i made i sewed elastic onto the top and bottom of the lining, just to make it stay a little more. i also thought about interfacing the lining, which i think would make it a bit more sturdy (which it probably needs; it is a garment most suited for sitting by a pool and not moving very much)