I found a cute tankini I had from last year in my closet that is a little too short now, but of course still fits up top. >.< I already bought a new swimsuit, but experimenting is still fun, right? ;.;
Grab your tankini and scissors!
Flip the tankini inside out~
Use your sharp object and poke a hole under seam of the bikini part...
Stick your scissors in the hole, and cut along the bottom of the seam, all the way around.
Use your scissors and clean up the edges if you need to.
Optional: Cut the seam in the middle of your back, then cover the edges with glue so you can tie it to make it fit better ^_^
... I didn't need to :P
If you want to, you can also make the rest of the fabric into a skirt by sewing up the edges with a bikini bottom attached :3
You could also add a pin or button to it as well. ^.^;