Step 1: Select Materials and Preshrink
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
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
Cut the curved parts out.
Step 5: Center and Pin
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
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
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!
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.