I had these scraps from a friend. They were already cut in convient strips from a quilt she made. As you can see, each strip is a slightly different shade of blue. The strips were 43" x 5". The width of the t-shirt was 10". So each strip was a little more then double the shirt front and back. For each strip I overcasted one edge so it wouldn't fray, and hemmed the other edge. Lots and lots of hemming. Then I sewed each strip in a tube.
Then basted and gathered the tube to fit the width of the shirt.
Then attached the gathered tube at the waist line. Next time, I would put pins to mark a straight line or use a washable pen to keep it even. I just eyeballed it and had to pick it out and resew it.
I measured about 2 inches down to attach the second tier. The placement doesn't matter so much, just that the bottom of the top ruffle covers the top of the second ruffle. (Also if your shirt isn't long enough to attach all 3 tiers, you probably could sew a tube that is the width of the shirt and the length that you want your dress to be. Then sew to your shirt and sew the ruffles on the tube.)
I'm not usually one to follow trends, but I think the ombre skirt gives it such character. I absolutly love how it turned out. It is just the perfect summer dress. It is so cute and girly, all full of flounce and ruffles.