this pattern by Garn Studios, and gave it a try. Hurrah! A rug for the porch!
If you want to make t-shirt yarn, there is a good tutorial at Mollie Makes -- this is just how I do it.
If you cut the strips wider, you get thicker yarn, and heavier t-shirts will make thicker yarn, but you can just cut them more narrow to compensate. Or not -- the difference would make your rug interesting. Which is what I say since I didn't compensate for the weight of the shirts! And, my other advice would be to use a quilt ruler and rotary cutter -- so quick!
To join the yarn, you can simply tie the ends in a firm knot then trim the ends, sew the ends together or do the fancy join I like best that is shown in this t-shirt yarn tutorial from dollarstorecrafts.com.
With a great big crochet hook, this rug was very quick to work up -- I made this one in one night, plus an hour or so the next day on the edging.