Video instructions and demonstration: https://youtu.be/D7o89HjmA0Y
Update (May 7, 2020): I made the standard version a little less wide and removed the "face up" and "face down" lettering. The measuring lines on the down side make it pretty easy to orient the pieces without the lettering, and it saved some printing time. I also changed the spacing between waves from 0.8mm to 1mm to allow for slightly-thicker double fabric. I also made a few other minor ease-of-use updates.
The STANDARD version is now 95mm wide (3.75 inches).
I also made an experimental SLIM version that should print much faster. That version is 75mm wide (2.95 inches).
The trickiest part in sewing face mask patterns such as the Deaconess and Pocket Mask is the pleats. This tool folds the pleats for you.
How to use:
Decide if you want STANDARD (95mm wide / 3.75 inches) or SLIM (75mm wide / 2.95 inches) and download the appropriate STLs.
Refer to Print Settings for recommended print settings.
This video explains it better than words do: https://youtu.be/D7o89HjmA0Y
Place the part with the measure lines on a sewing table, with the measuring lines furthest from you. Lay down your fabric and line the top edge with one of the lines. You'll need to experiment to discover which line works best for your pattern.
Place the other jig on top. Align the fins. Use your fingers to hold the bottom jig on the top corners and use your thumbs to push the top jig in, creating folds in the fabric. If the folds are too high or too low, try a different measuring line.
At this point, I use my hands to straighten the folds a little. Then I scoot both layers of the jig to one side of the fabric while I iron the other side. Then I repeat on the other side. You can either keep the fabric in the jig while you sew it, remove the jig and pin/clip the folds, or remove the jig, press the fabric with the iron, and sew without pins or clips. I usually do the third option.
If you don't need the measuring guide, you can simply download 2 copies of the "2of2" file.
This is a rebuilt-from-the-ground-up remix of Jonathan Wigger's pleat maker, which is available here: https://www.thingiverse.com/thing:4259706
Previous updates: (April 20, 2020): I completely overhauled this thing. It now prints in one piece* without supports. (*Well, two pieces since there are two jigs in a set.)
The "wave" shape of the new version leaves more space for two layers of fabric and just works better.
Recommended print settings, optimized for speed without sacrificing much strength.
Layer height: 0.2mm
Wall thickness: 0.4mm
Top/bottom thickness: 0.4mm
In Cura, check "Alternate Extra Wall"
Infill density: 13%
Infill pattern: Zigzag
Infill overlap: 16%
Horizontal expansion: To keep the actual size accurate to the model, you'll probably need a negative number in Cura's Horizontal Expansion setting. "-0.1" works for my printer. If you're having trouble getting two layers of fabric to fold in the jig, adjust your horizontal expansion on the next print.