This part is so stupid to explain but it's really easy. Sew the short edges (Kinda like surging) so they don't fray. Then flip it over and sew the long middle pieces together, wrong sides out.
Pull the diaper back through so it looks like this. The side flaps add absorbancy.
Then you're gonna want to sew the edges around the diaper so they don't fray. They're gonna fray every time you wash them for a little while, but just cut those pieces off and eventually once it gets to your stitches it will stop fraying.