As I have spent quite a chunk of time the last week making twelve of these T-shirt dresses, I thought I'd share my method.
3/4 yard of 45" fabric to 1-1/3 yard (depending on shirt size)
iron & ironing board
Measure your fabric. These were all made from children's size T-shirts. Two large, two medium, one small, one extra small, then 4T, (two) 24 M, 12 M and two different sized onesies--3-6 month and 0-3 month. Here are the measurements I used:
Large T-shirts--made from 1-1/3 yard (4 feet). Cut two pieces of 4 feet of fabric that are 10" and 12". If your girl is a little taller, go one or two inches longer on each piece.
Medium T-shirts--made from 1-1/3 yard. 9" and 11" pieces.
Small T-shirt--made from 1 yard. 8" and 10" pieces.
Extra small T-shirt--made from 1 yard. 7" and 9" pieces.
4T Shirt--made from 1 yard. 7" and 9" pieces.
24 M shirts--made from 30 inches, 5/6 yard. 6" and 8" pieces
12 M shirt--made from 30 inches, 5/6 yard. 5" and 7" pieces
3-6 M onesie (the one demonstrated)--made from 27 inches, 3/4 yard. 4" and 6" pieces
0-3 M onesie--same as the 3-6 M onesie
Cut the fabric. I measured all the way down and drew a line to cut along with a pencil. If you have a cutting mat, you won't have to draw the line with a pencil.
Step Three: Fold fabric so right sides are together and sew at the end 5/8" from edge. Then press the seam.
Press hemline by folding over and then folding it under the first fold and pressing.
Sew the hemline.
Layer the two pieces with the shorter piece on the inside. Right sides are both facing the inside. Line up at the seam with raw , unhemmed edges lined up.
Using a longer stitch selection on your machine, sew 5/8" from edge. It might help to pin these edges together. I did that on some dresses and others I didn't.
Stitch another gathering stitch with a longer stitch length just 1/4" from the first one, closer to the edge. (1/2" or 4/8" from edge). I find it's easier to gather the fabric if you use two gathering stitches instead of only one.
With right sides together, pin seam of skirt at seam of shirt. On the onesies, do this right above the leg hole opening. On the T-shirts, do it at the lower edge of shirt. Then gather the skirt until it matches the T-shirt/onesie width.
Pin the gathered skirt to the T-shirt all the way around. Make your stitch length smaller again on your machine and stitch 5/8" (to match where the 5/8" gathering stitch is) all the way around.
Finally, turn it right side out and you have a T-shirt dress!