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.