I recently acquired a serger and have been enjoying trying it out for the first time by sewing together some cloth diaper inserts.
I was given some flannel remnants that were too small for receiving blankets, but perfect for making some prefolds out of. They had to be at least 15" long for the right length to fit in the diaper cover. Then for the width, they could be anywhere from 35-50" wide. The first 30 inches, I folded in half with the right sides together so it was a 15" square. The remaining, I folded in layers 5" wide on top of the first 5" of the 15" square. Hopefully the pictures help make sense of my explanation.
Make sure that the 5" section is towards the wrong side of the 15" square not between the folds. In the picture above, the 5" section is underneath and the picture below it is on top. Depending on how much extra I had, I made 2-4 extra layers in the 5". The more layers, the more absorbent... and the more bulky.
Then I sewed all the layers together on the one side. In the picture above, I sewed the right side together with a 1/4" seam allowance.
Then I turned it right side out and centered the 5" section. Then to hold it together like that, I sewed along the seam and on the other side of the 5" section about 1/4" in. See the below picture to see the two places I sewed.
Then all that is left are the two ends. And I just went across that with 3 threads on the serger. So easy!
Here's one of them folded into the thirds.
I had one piece of flannel that was exceptionally large, I folded it in quarters (in half and half again) and serged the two sides with raw edges so that it was a 15" square. I didn't bother sewing closed the third edge so it can open there like a pocket and dry better. A 15" x 15" square of 4 layers folds into thirds for a super absorbent 12 layer insert.