Someone else commented that unused dryer sheets MAY cause damage to fabrics in the long run. In my opinion, used ones would work just fine. During the depression, newspapers were used and left in for more insulation. I would not recommend that. I have also used pieces of worn out sheets as foundations.
This method has been done for as long as quilting has been done. It is not at all new, but it is fast, fun and lively, as well as being environmentally friendly, as we use more of the fabric we buy.
BTW, you can use flannel, etc, but remember to leave larger seam allowances. The lamb print is a piece of flannel sheet.
My great grandmother (dead long before I was born) combined hers with no sashings or borders. I prefer to sash and border mine, so I do use some "new" fabric. The back could be another set of scrappy blocks or a regular backing. If you use sashings and borders, they will need batting behind them.