Here's what you need:
Size Q crochet hook
7-10 shirts depending on size. (might need more depending on size)
Pattern of choice or this leaflet. (which is what I used)
A good pair of fabric cutting scissors
First, you need to prepare your material into "yarn". You do this by cutting your shirts into strips. I found several methods for cutting t-shirts and joining the ends. Which I will talk about here. But, do also check out this method, which makes one continuous strip of material.
You do this by first laying the t-shirt out folded in half like this. If you notice, I've already cut the hem off. I save them in case I might need just a little bit more material. And if I do, I simply cut it in half. If you have a good pair of scissors, it's not hard to cut out at all and makes the procedure go by a little faster. I've even tried folding the shirt in half again but it's too hard to cut comfortably.
Oh, and I didn't think it mattered if the t-shirt had writing on it, but what happens is that the design especially if you used a lighter color will eventually show through. It really all depends on the shirt. Some printed shirts would stretch with the design on them, while others cracked and did not turn out well. You just have to figure it out for yourself.
I simply cut about 1-2 inch wide strips. I'm not too picky about measuring the strips. I just cut what looks like the right size and set them aside.
Then, I grab a hold of each end and stretch it until it won't stretch anymore. I repeat that process until I've got all the pieces stretched out. The thing I love about t-shirts is that when you stretch it, the ends curl inside and create a nice "tube". This means that your material won't fray. Not that it frays much anyways, but it gives it a neater look.
Shirts awaiting to be stretched:
I like to bunch up the shirts like shown above if I am making a colorful rug so I can see which colors look good together. But most times, I just start crocheting it grabbing whatever color I want and if I don't like the way it looks, I pull it out and start over.
After that, I use a square knot to hold the strips together. I was using another technique at first but found that the material slipped out and since this is a "rag" rug, I figured that knots were acceptable. When I crochet the rug, I try to get all the knots to the back anyhow. To tie a square knot, learn here. The reason I use a square knot is because it won't slip out. Here is a picture of my square knot:
The actual picture you see above is just an example. Had I used it, I would have snipped the frayed end off that you see. In fact, after I knot the ends together, I snip the material as close to the knot as possible. Once all my ends are knotted together, the "yarn" is now ready. Now, you just need to follow the instructions on your pattern.
Or if you don't want to have to worry about connecting so many strips together, follow this method to create one long continuous strip of material. You'll still have to connect the colors together, but will have way less knots to deal with. Click here.
Have fun crafting!
P.S. I will add that some t-shirts won't make good rugs. Sometimes the material will just fray. I've found that if you look closely at the t-shirt, if it looks like garter stitch it will work. However, if it looks like stockinette stitch, on the inside, don't bother. I made the mistake before I knew any better and bought a bunch of shirts with the stockinette look at a garage sale and was disappointed to find that I could not use them in my rug making. Remember also when choosing shirts, try not to get ones that have too much of a design on them. Even though your material will only show the inside, the printed part often makes the material "crunchy" and won't roll right.