To start with you will need to cut up old clothing into strips. You can cut up old t-shirts, leggings, socks ect. Use soft clothing (not jeans) that can be easily cut.
Don't be afraid of different textures. Also try to coordinate colors. For example, different shades of pink, purple and white; or red, black, and white. The strips should be about 1/2 inch wide and around 6 inches long. As you can see they don't all have to be the same size. I don't measure every strip, usually only the first one and then eyeball it from there.
Next you will need the base to your rug. What size shirt you use will depend on how big you want the rug to be. I would suggest a men's XXL shirt, I use a an XL or XXL. Cut out the neck, arms and back or front of the shirt. Trim the edges down to make an even square.
Using a ruler and a fabric pen (or if your cheap like me a sharpie) draw lines all the way across about 1/2 inch apart.
Next cut small slits about a 1 inch apart, following the lines as pictured below.
This part can be very tedious but hang in there, you are almost to the fun part!
Your shirt should look similar to this
Now remember those strips you cut up earlier? Take one and weave up through the first slit and down through the second.
Take another strip and weave up through the second hole and down through the third (it will share a hole with the first strip).
Take the two ends in the middle and wrap a mini rubber band twice around them. I use the tiny rubber bands made for hair.
Continue to weave and rubber band down the row, and then move on to the next line. This part takes a while but I find it relaxing. I watched two seasons of Criminal Minds while completing my latest one. Dr. Reid is my favorite character (he is the nerdy genious hehe)
After you have several rows done you can go back and take the strips on the ends that don't have a partner and rubberband them to the one on the row under it. And then you are done!