Sew around very close to the outer edge of your fabric with your turning hole in the front.
Turn through the hole.
Sew your elastic casings
Install your elastics and top-stitch to close up the turning hole.
Add your wing snaps (or velcro/aplix/etc.).
To use, fold in the soaker flaps.
They won't be longer than your smallest rise.
Templates can be downloaded here: http://prefold2fitted.blogspot.com/2012/10/online-free-downloads.html