Grab an old, baggy t-shirt. Or two.
Cut off the sleeves at the sleeves.
Save the sleeves.
For the top of the bikini measure from you collar bone to the bottom of your brests. Cut at about that length. (For width the side seams of the t-shirt should be fine.)
Take the top and hem (sew) along the blue dotted line. Remember to keep the sides open to pull the strings through.
Pinch and tie the middle of the top to kind of, in a way, make pouches for *ahem* the girls. Pull the strings through the sides and tie off each end. TaDa! Top!
To make the shorts/bottoms take what's rest of the shirt and put it on. Figure out where you want the bottoms to sit and cut up between the legs. (Don't forget to leave room on top to hem.)
Take one of the sleeves (you know, the one you were suppose to keep :))and cut the OUTSIDE part. You do NOT want to cut the seam! This will be the crotch area of you bottoms. Turn both inside-out. Then sew the edges of the "sleeve" to the cut edges of the "bottoms" with the pointed area pointing toward your butt.
Hem the top and the legs, then turn them rightside-in. If feel the need (like I did) put a string through the hem in the top to tie it snugly.