First, pic two t-shirts that would look good together.

On both shirts, lay flat and cut out as big a rectangle as you can, front and back sides.

You should have four rectangles.

On each piece (you can stack them and cut them all at once, or sparate for varying sizes)
Cut a diagonal line from top to bottom, leaving a smaller edge on one end than the other. If your shirt has a pattern than you want to go in a particular place, take in consideration that the smaller side will be the top of the skirt, and the longer side will be the bottom. (when I did mine, the words on my shirt ended up upside down, oops)
You can make a more dramatic difference between the short and long side by cutting at more of an angle on some pieces to a more funky look!

Now, lay your pieces out in the order you want them to be in. Sew right sides together.
Another tip, depending on your waist size and shirt size, you may not need to use all 8 pieces. I had big shirts, so I only needed four panels.

Since my skirt was a little big still, I added elastic. You can too, or you can add a drawstring, but both required you to fold the top edge on the inside and run your elastic or drawstring through. after you've finished this part, You're done! You don't need to hem the bottom since t-shirt material doesn't fray, but the pieces will be uneven, you can leave them as is for a ragged look, or cut them to the same length.

have fun and post your versions!