A flatfold is just a hemmed square of fabric. That's it. I like to use a serger to hem mine, and I round off the
corners as I sew around just to make the process faster.
The size of your squares is really dependent on your needs. Standard flats are traditionally 27" square - the
width of double-napped diaper flannel off the bolt. But that size may be too big for tiny newborns, and not
big enough for chunky toddlers. So, to customize the size of your flats, just double the length of your desired
rise measurement and use that for the size of your squares. Keep in mind that with the Oragami fold, the fit
will be very trim - bikini-style, so the rise won't need to be as high as with a fitted diaper.
When you're choosing the fabric for your flatfold diapers, you'll be looking for thin, soft, and absorbent. A
loose weave, like gauze, is a plus because diapers made from it can be used with a Snappi. The cheaper
grades of flannel also work very well, and muslin or equivalent fabrics may make a nice diaper, too. Try using
old flannel sheets, or see if anyone you know has spare lengths of fabric you can look through. (In my
experience, most seamstresses have boxes upon boxes of fabric that will never be used!)
Woven fabric can just be ripped square - snip it at the edge, and rip straight through. This will give you a
straighter edge than cutting ever can. If you have smaller pieces of fabric left over from ripping your squares,
you can fold & sew them into multi-layer diaper doublers to match your diapers. I make mine about 4" wide
and the length of the diaper.