Here are my instructions on how to make a prefold cloth diaper from a t-shirt:
I center an old diaper on the logo of the t-shirt solely for anal purposes, you can put it anyway that fits best and leaves a good sized strip for the center.
I first cut the strip for the center and take off the hem.
Then if you are blessed with a serger, have at it. If you are like me, use the large zigzag stitch and sew around the edges. Have one side of the zigzag stitch hop off the material and the other side go into the material, so that the edges are sewn together.
I then place the strip in between the two pieces of t-shirt diaper. I leave the extra length hanging out because of my choice to sew without pinning.
I then sew a straight stitch along both edges of the inside strip to tack it into place. I just use my fingers to feel where the strip is and I keep the diaper edges as close together as possible as I sew.
Then when it is tacked into place I go around the edges and cut them evenly where I have pulled while sewing or mis-cut so that all of the edges meet well enough for me to sew around the edge.
Here is the finished product. An Old Navy Prefold Cloth Diaper and Four Diaper Wipes.
If you would like to make it into a no pin adjustable prefold see further instructions.