Here is a picture of some of the supplies you will need- a flat front backpack (a pocket one will be harder), furry fabric, batting, E600 glue, embellishments such as rhinestones (I got a "G" for my daughter's initial), needle and thread, clothes pins, and ribbon or whatever else you want to put on the front. Don't laugh at my backpack choice, it was the only one I could find that was flat-front!
This is the batting I used from Jo Ann's fabric store. The thicker the better in my opinion!
Cut the fabric and batting in the general shape of your backpack. It doesn't have to be perfect.
Layer the batting on the backpack first, gluing it down in the center. Don't glue too close to the edges so you can fold the fabric around the batting to hide the batting.
Use your E6000 glue to tack down the fabric to the backpack, making sure to fold under raw edges of fabric around batting so that they will not be seen. I used clothes pins to clamp down the edges. Don't pull the fabric too tight so that the backpack can expand when it gets stuffed with school stuff inside it. Once the glue is dry, sew around the edges. I did just a couple tacking stitches- not the whole edge or that would take forever. It just makes it more durable. Or you could go around and glue the loose edges a second time if you can't sew.