This is my attempt at a tutorial...
Make yourself a pattern out of a folded piece of newspaper or something else (I like to use cheap plastic tablecloths because you can see through them). The dimensions for this pattern will give you a skirt sized small-ish, good for hips up to about 38 inches. The length is about 20 inches, give or take.
Unfold the paper, and lay your pattern out flat on a large Tshirt. If your Tshirt is too small, you will have to sew extra on the sides.... But it's all good.
After you've cut out these 2 main pieces (front & back of your skirt), take another t shirt and cut according to the diagram below.... You will see that all my strips are different colors, it's because I cut up a lot of different shits, so I have lots of random strips. When sewing, go right along the edge... No more than 1/2 inch seam allowance is provided!