I found this tutorial on Pinterest and went for it.
Step 1: I first measured the bottom of my mat, finding the width of my mat was 4 1/2", so I grabbed a bowl to trace that was just the perfect size and cut.
Step 2: Measure your mat. Use the dimensions of your mat and add 4-1/2” inches to get the length (1/2” for seam allowance & 4” for the top drawstring loop) and add 3-1/2” to the circumference to get the width (1-1/2” for seam allowance & 1” to allow room for your mat to slide in and out). My fabric measures 28-1/2” L x 16-1/2” W.
Step 3: Press the seams wrong side facing up, a ½” seam along the length of the fabric. Also press a 2” seam along the top.
Step 4: Sew the top, straight across the 2'' seam.
Step 5: Working on the right side of the fabric, pin the webbing strap to the fabric. The bottom strap should be 2” from the bottom & the top strap should be 1” from the top seam. After you pin, be sure to put the bag over your shoulder, or however you plan to carry, and make sure the straps are at the right length. Sew the two sides together, this will encase the straps.
Step 6: Probably the hardest part about the whole bag is sewing the circle piece to the bottom. Still working with your bag inside out, pin the circle piece around the bottom of your bag, pin, and sew. It's not the prettiest, but it'll do.
Step 7: Turn your bag inside out and marvel!
And what's a good Pinterest story without the embarrassing mess-ups? 1.) I sewed my 2'' opening completely shut so there's no hope for a drawstring (good thing my mat is snug as a bug inside it's new case and won't easily fall out). 2.) I didn't measure the strap, so once I sewed it to the bag, flung it over my shoulder, the bag hung way to low to be able to ride my bike with the yoga mat bag on my back. So like a crazy person, I just cut the strap in half. A not-so-easy, but less crazy option, would have been to rip the seams holding the straps and resew the side of the bag. I sort of couldn't believe I just cut it! But I tied a knot at the bottom and then safety pinned the knot to secure; the bag hangs just right on my bag, though a little hodgepodge-like.