Lay down the t-shirt and mark the lengh you want for your skirt. The maximum lengh is from the bottom of the tee to the highest straight line under the sleeves - so the rectangle shaped piece.

cut the line you just marked, than fold a little piece down and pin around the fabric.
Now try on the skirt. if it fits you well you're done, but probably it will be too loose around your hips.

If it is, fold it inside out, mark how tight you want it to be and saw down. Cut the excess fabric on the sides, than fold it back to the right side.
Your very new basic mini skirt is done. You can decorate it thousands of ways if you'd like, but it looks great as the way it is too.