Here is a tutorial for this top:
A big tee shirt. For this one, it should be a good deal bigger than your bust measurement, but exact measurements are not that important.
First, cut it across under the arms, for a large tube. Cut a one inch strip from the top. This is for the drawstring straps, which should be long enough. I had to cut excess off myself. But, if you're working with a shirt that is closer to your size and not so big as this one, you may need to cut some extra length from the top of the shirt.
Cut the underarms. For me, this is a 3" deep, 3" wide curve in the fabric when it is folded. You can use another shirt as a guide if you think this is not the right size.
cut a curve out of the front. mine is 2" deep, just a gentle curve from the center front to the top of the underarm.
So you see this is the blue version, but that doesn't matter. Fold and sew the edges of the underarms. Maybe there are more professional ways to finish an underarm of a knit garment, but this works, and it's good not to have too much bulk since the drawstring has to pass through.
Ignoring the zipper in the picture, fold both front and back top edges 3/4" under. It's not important to be exact, it just needs to be big enough for the drawstring. Sew 5/8" in.
Sew the drawstring. You'll have to cut it, since it is a big loop. turn it. It's a lot easier to turn it if you use a safety pin and crochet hook. If the fabric is stretchy or if the stitches tore while turning, you can stitch down the center to reinforce it. Thread through the front and back casings. This is easier with a safety pin and crochet hook, too. Fit it to yourself and sew the two ends of the drawstring together. you can arrange it so that the sewn part is inside the back casing, not showing.
I added a pocket to mine, which you can do if you like. I cut it from the sleeve so that the stitching would match. I was not exact with any of it, I just cut and pinned till I liked how it looked, and tapered it in 1/4" at the bottom so that it was slightly drapy on the top.
So yeah, there you have it! Enjoy!