1. Remove the button and any belt loops using scissors or a seam ripper, and cut the waistband off of the pants. 2. If the waistband has a fabric backing, trim it flush with the raw edge of the waistband and pin under both raw edges. Machine-stitch in place. If there is no backing, simply fold the raw edge of the waistband to the back and stitch it there. Set waistband aside. 3. Split the seam of the pants down the back and cut along the zipper as well, being careful not to cut too far into the pant legs. 4. Turn the right leg inside out. Pin it down flat so the seam is smooth and the cut edges from the front and back are close together. It doesn't matter if they don't exactly meet up or if the top edges are off. 5. Stitch a straight line up from the existing seam to the top and cut off the excess. 6. Turn the pant leg right-side out. It should look like a long tube. 7. Trim the top edge so it's even all the way around, and fold it over twice. Keep the folds small and make sure that the inside fabric from the pockets (the parts that were connected to the waistband) are tucked up there as well. Now stitch down the edge so you have a nice clean hem. 8. Grab your rolled-up yoga mat and slide it into the tube. Leaving a little room at the top, pin the bottom in a straight line at the end of the mat and cut off the extra material. 9. Remove the mat and the pins and turn the pant leg inside out again. 10. Match up the cut edges and re-pin. If one side of the pants has more fabric than the other, give it a pleat or fold so they'll lie flat. 11. Position the waistband (aka the strap) inside the bag so that the right side (the front) of the strap is facing the right side of what used to be the front of the pant leg. 12. Pull the end of the strap (the one without the buttonhole) through the pinned end of the bag so that the edges are lined up and the strap is equidistant from both side seams. Sew a straight line across from seam to seam, closing up the bottom of the bag and securing the strap in place. 13. Turn everything right-side out again and put the yoga mat back in the bag. 14. Pin the top of the strap in the middle of the top edge of the back of the bag, and sling it over your shoulder or across your chest (depending on how you want to carry it). 15. Adjust the length of the strap so it's comfy, being sure to leave at least 1 ½" hanging off at the end for the button tab. If you've got a lot of excess strap, pin it so that the extra length dips into the bag in a U shape with only the last couple inches reemerging. Take out the mat and stitch the strap to the bag. 16. Pull the tab across the opening and stick a pin through the buttonhole to mark where the button will go, then sew on said button. 17. Throw the mat in the bag, sling it over your shoulder and (if you're me) march into the living room, where there's no one to ogle you while you practice your best downward dog.