I made mine 'toddler' size, but basic size measurements are:
Preemie- 8 X 11 inches (cut fabeic 8.5 X 11.5) fits small babies & newbies to 10 lbs
Infant- 10 X 13.5 inches (cut fabeic 10.5 X 14) fits newborn through 15 lbs
Regular- 13 X 18 inches (cut fabeic 13.5 X 17.5) fits aprox. 15 lbs on up
Toddler- 15 X 19 inches (cut fabeic 15.5 X 19.5) fits aprox. 30+ lbs
Cut rectangular panels from the front and back of your cotton (or mostly cotton) sweatshirts using the specified measurements listed above for the size you are making. If you have a serger, you can cut to the first measurement listed, but for a regular sewing machine, cut the full size. Place right sides together and zigzag around (do not PULL the fabric through from the back, but gently push it through from the front...pulling will stretch the fabric and cause a 'lettuce' edging.)at about 1/4 inch from the edge, using a long stitch width and length (I used a 4 for width and a 3 for length on my machine) leave a couple of inches open for turning. Trim corners and turn, pin closed the opening and straight stitch closed at about 1/8 inch, just for the opening. Switch to zigzag and move over to a 1/4 inch, continuing evenly around the diaper, past the straight stitched opening to where you began to zigzag. Using the zigzag maintains the stretchiness of the fabric. Trim your strings, and done!
For doublers I usually prefer to do no more than 3 layers so that they do not take too long to dry. I cut each layer 5 X 14 inches, with rounded off ends, then overlock stitched around. If you do not have an overlock option on your machine, you can use the longest zigzag width you have with a shorter stitch lengeth (on my machine it would be a 5 for width and a 1 for length) Again, do not pull the fabric through, but push it.
This technique also works quite well with new fabrics...
Here I have used organic cotton and bamboo velour (first one is cotton, the other 2 bamboo) on one side, and a cute cotton knit print on the other.