This bathroom rag rug was crocheted with old t-shirts cut into strips. I did not have any pattern per say, but did a row of single crochet, crocheted into the back single loops on the way back, ending with 3 single crochets to start to form the oval shape, and continued around. I did square knots to tie strips of new t-shirt together as I crocheted.
I have to say this was difficult due to the elasticity of the material, and that using old sheets would be much easier. So if you are up to crocheting a rag rug, start saving your old sheets.