Intro: That-time-o-the-month Skivvies
How to add absorbent material to your undies and never buy maxi pads again, or toss them in landfills for that matter. This took me all of about 5 minutes. I like these a bit better than cloth pads b/c they are thinner, don't shift around, and are more comfy to bike with. But others have posted neat patterns for cloth pads, so look them up too!
Step 1: Materials
I use packs of cheap undies and kitchen towels from Trader Joe's. The towels are made of the same material as camp towels, I think, so they are soft, washable, and absorbant. You can use terrycloth, flannel, old socks, whatever you want, but i happen to like this combo. I use single ply in combination with a Moon Cup, but ymmv, so double or triple up if that's your steez. Of course, toss the whole shebang in the wash before you start to get out any sizing and/or cooties.
Step 2: Cut Absorbent Material
If using these towels, fold them in 4 and cut along the folds making 4 rectangles, then cut curves in the sides. Same idea using any other material, it just turns out that the towels fold into 4 pretty conveniently.
Step 3: Pin Fabric
Pin the towel into the undies at a place that makes sense for your cooter's geometry.
Step 4: Zig Zag In
Now, use a zig zag stitch along the side, and even along the top and bottom if you're ambitious. Zig zag is better than a straight stitch for stretchy spots, since it has some give. It turns out that my machine, a 1936 Singer 15-91 only does straight stitch, so I use a vintage zig zag attachment that actually works really well.
Step 5: Voila
Now zig zag down the other side and you're done!