Disappointed with commercially produced "100% cotton" cloth training pants that leaked because they contained, upon dissection, polyester batting instead of cotton, and searching for an alternative to "disposable" pullups, I made comfortable, breathable, wash-and-reuse pullup training pants, from materials I already had on hand.
Step 1: Select Materials and Preshrink
Select underpants that are big and loose on the intended child, but not actually falling down. Wash your intended absorbent layer and the underpants, in the hottest water you can, and dry on high heat, to shrink everything all it can possibly be shrunk, before beginning.
For the absobent panel, I repurposed tattered old cotton prefold diapers that had fulfilled their first duty as diapers for multiple babies and were approaching rag status. If you have no prefolds, or yours may either be re-used or sold, you might find ones in ragged condition you can get for nothing, or close to it. Otherwise, you can substitute anything you think will hold enough moisture, that your sewing machine can stitch through the layers of.
Step 2: Gather Necessary Tools and Put in a New Sharp Needle
Make sure you have pins, good fabric scissors or else a rolling quilting cutter and cutting surface, because you'll be cutting through many layers at once, something to mark lines (doesn't matter what, really: crayon, chalk, pencil, magic marker), and of course a sewing machine capable of quilting, with a zigzag stitch.
It's always a good idea to put in a new sharp needle with each project, but because this one may push the abilities of your machine to punch through so many layers at once, a brand new "quilting" needle is a good idea.
Step 3: Cut the Middle Panel Out
Cut the smaller side panels from the prefold, leaving the thickest middle part. [Side panels can be used for another project, even cloth feminine products]
Step 4: Mark and Trim
Turn underpants inside out, and hold the crotch onto the middle panel of the diaper, and mark the curvature so that the insert will fit perfectly in the crotch of the underpants.
Cut the curved parts out.
Step 5: Center and Pin
Center the insert in the crotch of the underpants, with the curvatures matching the underpant legs. Pin in place from the crotch outward. If the ends of the diaper insert are too long, trim them to make the insert short enough to fit within the underpants and not hang out beyond the waistband.
Then center the ends of the diaper insert, to the middle of the underpants. The tag is usually a good reference point. Pin the midpoint right there at the tag for reference, and then pin outward from there toward each end of the diaper insert. That will keep it straight with respect to the pants.
Step 6: Stitch Legs From the Center Out
With the zigzag stitch set really wide and really short, which mimics in some respects a serging stitch which covers the raw edge while providing for stretch, start stitching one edge of the insert, from the CENTER of the crotch, out toward the waistband. Then turn and stitch out from the center again, toward the other waistband, and repeat the same maneuver for the other side.
This is the same method used for collars and necklines on garments, and it avoids creeping and puckering, keeps things aligned.
Note the zizgag encompasses the raw edge, almost like a satin stitch.
Step 7: Tug Elastic to Gather and Stitch Tops
Once this is done, you can trim off any excess absorbent layer at the waistband, if there is anything sticking up, before beginning to stitch the tops near the waistband.
Again, you will find it useful to begin stitching in the center, out to the end, then turn it around and stitch from the center to the end again, to avoid shifting. But this time, you will want to pull the elastic of the waistband out just a little while you stitch, so that the absorbent layer will gather a bit with the waistband. That way, the waistband will still be stretchy, and the absorbent layer will move with it.
Alternatively, you could trim the end of the diaper insert (the absorbent layer) so as to be stitched well below the waistband.
In the photo below, you can see that this diaper didn't need trimming. The original navy-blue serged edge is still intact. This is not from my machine. I am stretching the elastic a bit as I begin the zizag, right over the serged edge.
In the second photo, I am turning it around to stitch from the center, out to the other direction, again pulling a slight bit on the elastic as I sew, as if gathering.
Step 8: Wash and Wear!
The finished item, inside, and out.
If you preshrunk everything properly, these are ready to wear! I found them to be useful, thrifty, and comfortable for the child, as an alternative to disposable training pants / pullups, using materials I already had on hand.