For all of you who gather the skirt fabric to the bottom of the T-shirt, phooey!
Now that I got that out of my system, you know I'm going to tell you how I do it.
The first time I did this, I had a T-shirt I had bought from our local ASG chapter which has an emblem printed on the front. It has "Sew America - September is National Sewing Month" printed on the front in pastel colors.
I found a 44-inch-wide madras plaid cotton upstairs (a.k.a. attic) in pink, yellow, blue, purple, etc., which goes with this perfectly. I had exactly enough of this to cut approximately in half and match the plaids at both selvedges and stitched together so it formed a loop. Cut the bottom and the top off evenly, and hemmed the bottom.
Turned the skirt part inside out and began to attach the right sides of the fabric and the T-shirt together. Matched and pinned the seams to the "side seam areas," halved and pinned that to find the center front and back, and continued to half and pin until the pins were about 3 or 4 inches apart. Started stitching at the near side of a pin, stitched almost up to the next pin, pulled the excess fabric towards me, forming a pleat. Continue this all the way around, pulling out the pins just as I got to them but after forming the pleat with the fabric.I find it easier and neater looking than gathers.