So... Below, I will step you through what I'll call the "Slit-Opening Pocket Diaper. :)
The only pieces you'll need all prepped and ready to go.
Make your slit large enough to easily stuff. If you're sewing an aplix diaper, then cut in between the two laundry tabs.
Attach faces-in before sewing.
Here is an OS Squaretab all ready to go as well.
Straight stitch about 1/4" in from the edge all the way around the outside. Then trim off the sharp corners before turning.
Turn through your pocket opening. I then stitched this one with ruffled legs and back by top-stitching all the way around about 1/8" from all the edges BEFORE sewing my elastic casings.
Here is a close-up view showing the simple pocket opening and the top-stitching.
Then sew your elastic casings along the legs and across the back.
Install your elastics through the pocket opening, and attach your wing snaps (or aplix tabs).
I then used a one-size Bumgenius insert that I added a 4th snap to (to match the rise options in my template). Cottonbabies.com occasionally has used insert sales where you can snag inserts for about $1 each when bought 4-6 at a time.
Stuffed at full rise.
Both the insert and the diaper set to the second smallest rise.
And the NB-S Squaretab:
Templates can be downloaded here: http://prefold2fitted.blogspot.com/2012/10/online-free-downloads.html