Step 1. find an old/stained tank top. (or t-shirt).
Turn it inside out, fold it in half so the side seams are together (on the left in the photo below)
Use the existing hem, and cut the tank parallel to the hem as long as you want the shorts to be (plus 1.5-2 inches)
Cut a J, about 1.5 inches wide, and 2/3 of the height (you can see I started with a small J, and kept making it longer and longer.
Open the tank flat, and sew the Js together.(yellow stitches. I used my serger, but if you don’t have one, use a stretch/ball point needle, and a zigzag stitch. Or don’t, just leave the edges raw.)
Cut off the remaining side-seams (the part left at the bottom of the J), and line them up, with the J seams straight. (hmmm. I think the picture above is WAY more clear than my words.)
Sew those raw edges together (if you have a serger, these are the threads to tuck in, so you don’t get itchy edges).
Turn right side out. Fold over the top, to the inside, and use elastic thread to shir through the fold, gathering the waist. I went around 2.5 times, and then my bobbin was empty, and the Bean didn’t want to wait for me to wind a new bobbin. So these aren’t tooooo shirred, but they’re shirred enough to stay up. 
Because I’m sewing for a 2 year old and the tank is an adult medium, they’re going to be roomy, so perfection in size doesn’t matter. You don’t need a pattern for something this simple….