I have made several of these t-shirt dresses for my girls. They are the best play outside dresses! Put some leggings underneath, and there is a lot of room to run and play, climb trees, turn cartwheels, chase chickens...
A T-shirt in your child's size
Fabric of choice ( 1/2 yard for 12-24 months & up, 1 yard for 2T and up)
Rotary cutter, rotary cutting pad, ruler (optional)
Begin with a t-shirt in your child's size. For an infant/toddler, I measure 1.5" down from the armpit of the shirt on each side, and cut straight across. For 3T and up, I measure 2" down from each armpit. You can use a rotary cutter and ruler to cut, OR you can simply mark a straight line from one end to the other and then cut with scissors. Your shirt will look like the above picture.
For your bottom piece, you want to measure the circumference of the bottom of your shirt. Then double that. This will be how wide you want to cut your fabric. For instance, if the circumference of your shirt is 24", you will cut your fabric 48" wide. If you want it more gathered, you can measure the circumference of your shirt and multiply times 2.5. You may need to cut two separate pieces for a size 2T and up.
As for the length, I put the half shirt on my child, and measure from the bottom of the shirt to the length I prefer. When you get that measurement, add 1" to it. This will give you a 1/2" seam allowance.
Now you want to sew up your short edge(s). You may have one edge to sew up if you are making this dress for a little one, but you will most likely have two pieces for an older child. Sew each short side together and serge if you prefer.
For the top part of your skirt which will attach to the bodice of your shirt, you will need to gather. I gathered mine on my serger, if you don't have a serger, you will need to run two basting stitches around the circumference of the top of your skirt, and then pull the stitches creating a loose gather all the way around. Don't gather too much, you will need to adjust the gathers to fit your shirt in the next step. You can find a tutorial here.
Then you place your shirt RIGHT SIDE OUT inside the skirt piece. Your skirt piece will be inside out, and your shirt will be right side out, therefore you will sew right sides together. You need to pin the shirt and skirt together at each end on the seams. Then adjust your gathers to fit the base of your shirt.
Be very careful not to pull the shirt, knit is pretty easy to stretch when your working with it. You may want to add some extra pins to pin your skirt to shirt so it won't shift while sewing together.
Next, sew the bodice and skirt pieces together (1/2" seam allowance) being careful not to pull the shirt as you sew. If you ran basting stitches to gather your skirt, you will need to pull those completely out once you've finished sewing the two pieces together.
Your dress should look like this so far.
The last step is hemming your dress. If you have a serger, serge around the edge, then turn up 1/2" and sew the hem down. If you don't have a serger, turn your hem up 1/4" iron down, and then turn it up 1/4" again, and iron down. Then you can sew the hem down all the way around.
My daughter wanted to add lace to this one, so after she turned up her hem, she sewed the lace onto the hem at the same time securing/stitching the hem. Be sure to sew your lace to the inside of your dress hem.