As you are probably aware you cannot cut into a sweater without the stitches coming undone. If you are determined to use sweater material, and I can see why you would want to, please use an iron on interfacing and iron it to the back of the part of the sweater you want to use ( big blocks) before you cut anything. Make sure the iron on goes beyond the edges where you want to cut so that all edges are stabilized. Then you can use them the same as t-shirt blocks and make a quilt. I hope you can find an experienced quilter who can help you through the project. You might also want to stitch close to the edge of every block before you assemble it into a quilt so that it is impossible for nay stitches to get away. Good luck! If you use sweaters, I don't think you will want batting in your quilt.