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.
To pick up stitches: first do a rib swatch with the yarn you'll use for ribbing. A good rule of thumb is to take recommended gauge and multiply that times the circumference of the opening of the mitten: IE: If it is 8 inches around and the gauge is 5 st/in on #7 needles. Figure 8x5=40, but drop down the needle size one to two. Adjust up or down depending on stretchiness of yarn: It has to fit over the fat part of your palm and come back into shape at your wrist.
To pick up stitches: mitten right side out, mark out 2inch incriments on opening of mitten, and with a sharp knitting needle and holding yarn on the inside, poke through however many stitches per inch you need: IE 5/in= 10/2in. Use needle as you would a crochet hook (or use a sharp crochet hook). Make sure you are creating these stitches to the inside of your sewing firmed border. I like to do cuffs in the round with double pointed needles so: Do your rib stitch round and round to desired length and bind off in the stretchy cast off of your choice. Viola! Felted mittens. In a few wearings, they will mold somewhat to shape, especially if they get wet in the process. If you want to be tricky, you can cut, pin, and stitch a slightly smaller palm side to a larger top side in especially firm felt. Experiment, have fun. PS: Superwash wool on the cuffs makes them machine wash and dryable.