So... firstly, you can do fitted assembly in several ways. Sewn-in petal soakers, wet-zone soakers, snap-in soakers, trifolded thin soakers. LOTS of options. Below is a photo of both my NB snap-in pieces and my inner absorbent fabric + wet-zone soaker layering pieces.
For this tutorial, I will be doing the wet-zone soaker. I start by just zig-zagging my soaker in-place on my inner fabric layer.
Then I serge around the outside of all but the front/top of all my layers.
It ends up just like my turned & top-stitched diapers with the opening at the front.
Inside showing the wet-zone soaker.
Next I sew my elastic casing channels. The back elastic...
Then the leg elastic.
The outside showing the elastic casings.
Inside showing elastic casings
I use a bodkin to thread my elastic and tack it into place, but I used to use a safety pin.
Stretch to desired tension and tack the other end. Then trim.
This is how I hold the leg elastic tight before tacking the second side.
Now all your elastic is installed.
All that is left is serging closed the front opening and adding the final few snaps.
Serge across the opening starting about an inch back.
It will look like this:
I then use a yarn needle to thread through my serge tails.
Then you just trim off the little bit that is sticking out. :)
Add your top umbilical snap and your wing snaps, and you're done!
Here is the inside showing the wet-zone sewn-in soaker pad seam lines.
Templates can be downloaded here: http://prefold2fitted.blogspot.com/2012/10/online-free-downloads.html