The pattern I used was for the 3D/origamimasks. Lately, I’ve found these to be the most comfortable. The elastic measured 8 inches long prior to being sewn onto the mask.
1. Just to reinforce the sides, I folded them as I normally would a channel, and placed the ends of the elastics about 1/4in inside each corner.
2. After pinning, I set my stitch length at 4. I put the mask through the long way to get a little bit of leverage to pull the bulk of fabric through under the foot.
3. I sewed across the elastic on both sides, then top stitched over the open channel seams. Done!
The flat elastic made it easy to attach and sew. The toughest part was just orienting the elastic so the ends face the same direction. I didn’t have to move the adjusters at all, and there’s still a bit of room for movement, if needed. Overall, I would be willing to use them again since it saves me the time of measuring out my own elastic, and working the adjusters on myself.