Keep in mind, as the t-shirts are layered, this rug results in a somewhat uneven floor surface and is not well-suited for people that are accident prone or generally have trouble walking.
Step 1: The idea
(I drew this diagram to sell my very skeptical girlfriend on the idea)
Step 2: Collect T-shirts
If you still don't have enough shirts, you may want to see if the regional Goodwill Store has a "last chance" outlet. The shirts found at the Goodwill "last chance" processing depot will be remarkably cheap and are usually moments short of making their way into a dumpster. You may as well put these shirts to use.
And so you ask, "How many shirts will I need?"
My space was 9 x 12. I used roughly 20 - 30 t-shirts.
Step 3: Start Laying them out
Step 4: Pin them together
Remember to leave a little bit of space at the ends so that should later your want to slide one shirt under another, it is not sewn down.
Step 5: Sew!
Remember not to sew entirely to all of the outside edges in case later you want to slide one shirt in-between two other shirts.
It helps to start from the outside edges and move towards the center. Any which way you do it, if you're making a sizable rug, sooner or later it will turn into a seemingly unmanageable mess of fabric.
Don't forget to sew down the neck holes on the shirts or people could trip and hurt themselves!
Step 6: Lay down more shirts
Continue this until you have enough shirts covering the entirety of the area you are looking to cover.
If you are happy with an unkempt t-shirt in every direction look, then you are obviously done adding shirts. You just have a little more work laid out ahead of you.
Step 7: Finishing touches.
Also, trim any excess thread hanging off the shirt if you haven't done so already.
You might consider vacuuming your rug to pick up any loose threads or other schmutz. I have yet to try to send it through the washing machine, but that should be fine as well (I hope).