Here are some tutorials:
http://seamstresserin.com/sweaters-blanket-tutorial/
http://yellowsuitcasestudios.com/201...nket-tutorial/
http://www.instructables.com/id/Cash...uilt/?ALLSTEPS
Here's a Youtube video that shows some examples (no sound):
https://www.youtube.com/watch?v=2wBIG_LRgqY
The last tutorial shows how to use strips of fusible on the edges so the seams don't stretch. I like that idea. You would not have to buy pre-cut strips, although the strips sold to join battings would work. You could also cut your own strips out of fusible nylon tricot or other fusible. I would experiment with ironing the fusible onto scraps of the sweaters first to make sure it will adhere properly.
Instead of strips, I would be inclined to see if a very lightweight fusible could be used on the entire back of the sweater pieces. Once the sweaters are de-constructed (there are Youtube videos and online tutorials in how to de-construct a shirt; a sweater would be similar), I would iron the fusible on the sweater pieces before cutting into the shapes for the quilt. This is the fusible I would be tempted to try first (can be purchased in smaller sizes; I think Walmart and JoAnn's sell it by the yard):
http://www.amazon.com/gp/product/B00...=sr_1_1&sr=8-1