Summary: Elastic cuffs are formed by sewing in the elastic while keeping it stretched.

1. Prepare your materials: Start out by ironing in a hem with a width the same as your elastic. Fold it twice so that the raw cuff edge will be enclosed in the fnished hem. Size the elastic by wrapping it around your wrist, stretching it to the desired tightness, and marking with a pen. Make sure there's at least two inches of excess elastic 'tail' on each side of your wrist mark. Lay down the elastic and make another mark one seam allowance to the outside of your two wrist marks. These outer marks are what will be visible outside the hem when you stretch the elastic during sewing.
2. Sew in elastic: Insert the elastic into your hem and line up the outer marks with the sleeve edge. Measure in one seam allowance and make a small mark on the hem to designate where the two bartacks will go. Insert the fabric and elastic into your machine and secure with a bartack at the mark you just made. Leave the needle down in the right side of the bartack, then raise the pressure foot and reset the machine to a regular stitch. Lower the pressure foot, then grab the two elastic tails and stretch the elastic until you can see just see the outer elastic mark through the hem on the far side. Then, keeping the elastic stretched, run a line of stitching to secure the elastic. When you get to the other side, stop with the needle down in the bartack mark. Raise the pressure foot, set your width and stitch length dial for a bartack, and secure with seven sitiches. Stop with the needle in place for the 2nd line of stitching, raise the pressure foot, rotate the fabric 180 degrees, reset the dials for a regular stitch, and then lower the pressure foot. Restretch the elastic and finish off with a 2nd line of stitching.

3. Finishing up: Peel back the hem and trim off the excess elastic. You want to keep the bulk of the elastic out of the finished seam. Add a bartack to reinforce the seam at the cuff.