They're of a soft thin cotton fabric, with embroidery thread to tie around the waist. The fabric is teal, but looks more blue in the photos. I'm using a webcam for the photos in this post rather than my phone, so the pictures in general will be uglier.
Below the cut is an explanation of how to put the wrap pants on, and the pattern so you can make your own.
(The shirt in these pictures is the undershirt from Eun-jae's very first post.)
Flip her over, grab the threads on the back panel, and tie them together on her stomach. Done!
If you prefer a visible bow on the back rather than the front, tie the back panel on first instead.
Go around the edges with a blanket stitch or Fray Check. I've never owned any form of Fray Check (probably should...), so blanket stitch it was. (If you follow the pattern below, you'll end up with 1/5" seam allowance and 1/5" hem allowance, much less than I used. Feel free to change it.)
Sew the crotch (curved bit) together, clip the curve, hem all other edges, and tie embroidery floss or ribbon to the corners as shown in Craftgrrl's tutorial.