For this option with the fold-over-elastic (FOE) along the top, cut without the dashed areas.
Start with your inner fabric (you can sew in your soaker/absorbency or make it pocket-style) and your PUL laid faces together as you do in order to turn and topstitch (T&T). Just sew very close to the edges as shown below though, and not all the way to the edges nor secured at the back. All the trainers have stretchy hip panels which start with 5 inch x 6 inch rectangles folded in half to be 5 inches x 3 inches.
Fold the back sides inward. Make sure they are at least 2.5" high (to match up with the front sides).
Sew your leg elastic casings (I use 3/8" poly braided elastic):
Here the leg casings as well as the hip elastic casings are shown.
Install your leg elastic. I leave it much looser than I would for diapers. About half-stretched.
Top-stitch and install your FOE stretched loosely across the front.
I use a tight zig-zag and fray-check on the ends of the FOE.
Install your elastic into your hip panels. Also not pulled very tight.
How I add the hip panels into the body:
Make sure your inner and outer fabrics line up on either side of the hip panel before pinning.
Use your top-stitching to attach the hip panel pieces.
I run a second parallel seam to strengthen.
Trim the top to line up with the back and where the front will connect as well.
This is how it should look all cleaned up so far:
Install the FOE, stretched to about 3/4 stretch, all the way across the back.
Finish off the FOE ends and add your snaps.
You can do another set of snaps set further in on the hip panels as well.
If you chose to do a pocket, there would be a pocket opening on the inside. ;)
Pull off and on like underwear, but can also be opened from the sides for any poopy messes!
Here is one I made with a flap-style pocket opening at the back: