The first thing that you need to do in order to transform a shirt, a t-shirt to be more precise, into a back pack is to choose a t-shirt that you want to use for the purpose it is best to choose a t-shirt that has no image or printing on it. There is a very good reason for avoiding a t-shirt with an image or printing on it. Once you are done making the pack back, the image or printing will be upside down, thus making it looking really bad.
After you have chosen a t-shirt to turn into a back pack, cut off the sleeves as well as the bottom hem. You basically cut across the t-shirt from arm-pit to arm-pit. Make sure you do the cutting part carefully, keeping the cutting as straight as possible. If you see no point in cutting off the sleeves since you are going to remove that part from the t-shirt anyway, there is no need to waste time cutting them off. Just do whatever seems more convenient to you.
Once you have cut across the sleeves and the bottom hem of the t-shirt, turn it inside out.
Use a thread and needle to sew the arm holes, in case you did cut off the sleeves. Also stitch the bottom of the t-shirt closed. Stitch just half an inch in from the boundary of the cut. This stitched end will serve as the bottom of your back pack.
Keep the t-shirt turned inside out and hem the top of the t-shirt just about an inch. Make sure you leave an opening of about an inch in the middle of either of the two sides.
After you are done with the hem, get down to cutting a piece of drawing-string, just about 75 inches in length.
Remember the 1-inch opening that you left while hemming the top of the t-shirt? Use it to thread the draw-string through the top hem.
You are nearly done. Just sew the ends of the draw-string either of the two bottom corners of the back pack.
Now turn the t-shirt right side out and pull the draw-string to close it.