Felt an old sweater, or the use the one accidentally felted by your "sweetheart". This is a very good use for those wool sweaters that have moth-holes you don't or can't fix. Top stitch over any holes with sewing machine to stabilize fabric before felting. You will fold felted fabric, and the fold will become the thumb side of the mitten. Mark and cut a mitten shape sans bottom cuff, DO NOT CUT FOLD: The mitten will be more comfortable if there is no seam on this side. Same theory, cut a thumb shape with the fold on the inside of thumb, not forgetting to have a little extra ease, especially at the bottom. In the folded side of the mitten, slash an opening where the thumb will attach. A slit about 2-2 1/2 inches should suffice. Pin and stitch the thumb to the mitten using a short straight stitch. (If the felt isn't terribly firm, do two rows, close together: You want a small seam allowance. Wrong side out, stitch down outside edge of thumb using same technique as above. Straight stitch and zigzag, or serge across bottom of mitten to secure fabric structure. (Do not stitch the bottom of mitten closed.)
Wrong side out, Str Stch and zigzag (or serge) the top and side of outside of mitten closed.