Thanks to Stephanie for this Frugal Baby Tip!
I used Premium regular size prefolds from www.greenmountaindiapers.com. You could alter the directions slightly for a smaller or larger size. I plan on buying a few premie and infant sized to alter for the next baby. The regular is just right on my 8 month-old 20 lb baby with lots of room to grow.
1. Start by tacking elastic along the top/back under the surge line. Zigzag tack 1/2 from either side stretched to outer edges of center layers. Stretch as much as needed to meet your needs. I think I've been using about 1 1/2 to 2 inches. This give a side stretch. I've found it stretches better than along the center section due to the thickness difference, but if you want to try it in the center, go for it!
2. Add in snaps (non-poky side) or Velcro to both sides along the same line as elastic, about 1/4 to 1/2 inch from outer edges. If you don't want closures you can leave this step out and secure with pins or Snappis. I zigzag stitch the elastic and snaps. I am prefering prefastened snap strips. Pull it apart and put 3 snaps on each side, sewn onto the very outer edge so the strips stick out like the tape/Velcro of disposables. You would sew the Velcro in the same way. (With Velcro, I would add fold back tabs.)
3. Fold over, and while stretching out to full length, carefully sew a seam following the surging line already on the prefolds. Be careful to not sew onto the elastic.
4. Rotate to bottom/front of diaper. (If not using snap strips or Velcro, skip to step 5.) Sew Velcro or snap strip (poky side) to outer edge at desired height, leaving room to fold down and stitch. I leave enough room for folding the edge back over to allow a good inch or 2 from snap line to top edge. (I just measured it at 3 1/4 inches from the raw edge.) I use 6 snaps across the middle section. (My snap strips were too narrow to sew nicely along outer edges, so I zigzag stitched vertical lines along outer edges and between each snap.)
5. Fold down/in about 2-4 inches depending on size for prefold andbaby. Fold corners of flap in/under. Sew straight seam along all 3 folded edges. (Looks kind of like a sealed envelope.)
6. Mark for side elastic gussets by folding sides in so the outer edge is flush with the closest inner section seam. That fold is the line you want to tack the elastic to. I use a pencil to mark the inside. (It's actually easier to measure out before sewing.) I make 2 marks about 6-7 inches apart, one near top of dipe and one near bottom. I tack the elastic (1/4 inch) on the mark closest to the back of the diaper. Don't cut elastic, yet. It's easier to do the next step without having to stretch the elastic out while sewing.
7. Fold the side back in with elastic on the inside, then fold over again so the second fold brings the material over into the middle section of dipe. The elastic is now hidden inside the fold. Then I sew a strait seam 3/4 to 1 inch from the outside fold, being careful to not sew in the elastic. Go the full 6-7 between both tack marks. This sets up your gusset. Pull eastic until you reach desired stretchiness, then hold tight or pin and zigzag stitch over tack spot near fold. Cut elastic or see step 7 for alternate idea.
8. If you want to be fancier, you can stretch again and tack down right under front side corder for added stretch.
REPEAT steps 6-8 for other side.
9. For a more finished look, sew along folded in outer edge from gusset seam to outer corners.
It sounds complicated, but it's not bad. The great thing is since you are not cutting the diaper at all, you can use a seam ripper when you make a mistake. (I've ripped a lot of seams so far.) My biggest tip is to remember to drop your foot completely each time.
It is often thick enough that it feels tight even without the foot dropped. But when I don't drop it, the machine "pukes" thread all over the place. I can try to take pics as I go with the next one, so you can get a better feel. You could also just do the gussets and fold in the top and bottom each time you fasten to baby. That would still help contain the poopies and make changing a little quicker.