Are the legs wide enough for your yoga mat? Long enough with some extra space for seams? It's important to verify this before we go any further.
Step 2: Cut the tube and straighten it
Even straight-leg pants have some change in shape along their length, so turn the leg inside-out and pin and sew the flared part(s) to make a straight tube. Cut off the excess fabric leaving a reasonable seam allowance (1/4 to 1/12 inch).
Step 3: Create a strap
This bag's main purpose is to allow you to carry your mat on your shoulder, so a strap is essential. Cut a length of fabric from the as-of-yet unused leg of the pants to match the length of your tube, and fold it over in a manner that hides the fray-able edges. Sew.
Step 4: Sew the bottom seam
We're going to create a pleat at the bottom that will make a less boxy shape at the end of the bag. with the tube still inside-out, fold the edges in on themselves like in the picture, and sandwich one end of the strap into the bottom seam (with the long side inside the main tube). Pin and sew (you may want to sew several times for reinforcement).