We pinned (as in Pinterest) here and here, so you should know this is no original tutorial, we're just sharing...
Here's our specimen on the operating table.
Yes, I'm corny as all get out.
We started by cutting off the sleeves.
Next, we made a straight cut under the neckline and shoulders.
We rolled the top edge down and pinned, doing the same to the back. We stitched each of those down with a long straight stitch.
We cut 1 1/2 to 2 inches off the bottom of the shirt. Then, cut again right above the hem. (This'll be used as our strap.) Don't forget to snip each side so you have two long strips instead of one long loop. And since knit doesn't fray, all you need to do is stretch your strip and it'll curl.
Using a safety pin, we fed each strap through the casing we made.
(Remember, that front and back edge we sewed down?)
Yep, feed it through there and tie!