Make an individual crochet in the second chain from the hook and in every chain across. As you make further chains, slowly increase the width of the rows by creating two single crochets in the first and last stitches, as well as a crochet in each stitch across the middle of the row.
Continue the above process until the mitten is long enough to reach the tip of your longest finger from the bottom of your palm. If the mittens are for someone else, you will need to either measure their hand length or simply guess it. Fasten the yarn and cut it, leaving a trail of about 10 inches. Repeat this entire method to create the other side of the mitten.
Use your crochet hook to make a slip stitch in an effort to attach the yarn to the bottom of the mitten. Make a single crochet on the edge of every row. These should join at the end with a slip stitch, forming a circle. Chain a single crochet only on the front loops until the cuff is as long as you desire. This length should be around three inches. Then, weave in any loose ends.
Lastly, attach the yarn into the opening of the thumb hole with a slip stitch. Make single crochet stitches around the opening, joining it with a slip stitch. Repeat until the thumb reaches the desired length, and then single crochet two stitches together around the thumb. Cut the yarn, thread it into a needle, and pull tightly to close the thumb.