My brother who is 6'10" left a t shirt at my house that he didn't want. i used this shirt that would have fit me twice to make the comfiest pair of yoga pants ever. Sorry no step pics, i wasn't sure it would turn out.
Cut the collar of the shirt out.
then cut off the sleeves leaving a groove where they were.
(Save these parts)
Cut all the way up the sides (Not middle) until you have 2 sheets of fabric. Fold each section in half wrong side out matching up the sleeve grooves.
Place the folded sections side by side so the sleeve grooves on each piece are facing each other. this should look like a pair of pants.
Take one section and sew with a whip stitch from the bottom (Straight Edge) to the bottom of the sleeve groove. do not close the sleeve groove. this will join up with the other side to form the crotch of the pants.
Repeat on other section of fabric.
now you should have 2 inside out tubes of fabric. now join them by sewing, again with a whip stitch the joined sections of the old sleeve grooves from the tops of both all the way around to the back.
now fold the top of the pants over still wrong side out about 2 inches and sew around the bottom edge using a running back stitch. be sure to leave at least a fingers width of room inside the seam. this will be where your draw string goes.
once you have that done flip the pants right side out and cut 2 small holes in your draw string seem about 2 inches apart. i put mine on the front but i have seen it on the side and it looks cute.
cut a long strip of your leftover fabric (Your waist + 8 inches) attache a large quilting pin to one end of the strip and begin working in threw one hole an out the other.
TA DA! you have yoga pants!