Adding the Cuffs
Cuffs are always self-facing. There is not a lot of difference between attaching them and attaching a self-facing collar. The difference is mainly with the sleeve itself, rather than the cuff.
What is a gusset?
A gusset is a fabric insert at the underarms of sleeves that are cut kimono-style, that is, cut all of a piece with the body of the shirt or dress. Gussets make the shirt more comfortable by allowing more arm movement.
Preparing the Cuffs
Prepare the cuff in much the same way as you did the collar. The outer cuff piece will be lined with interfacing. Press under the seam allowance on the notched side of the inner piece. Sew the pieces together along the three unnotched sides, clip the corners, grade the allowances, turn, and press.
If your cuff is one that folds in half, rather than being cut in two pieces, the notched half should be lined with interfacing and the opposite end turned under. This cuff might be sewn onto the sleeve before it is folded.
Before you can attach a cuff, you need to sew any tucks or pleats that your pattern calls for onto the lower edge of the sleeve. If there are no pleats or tucks, the sleeve will likely be gathered.
Sew the sleeve seam, and press it open.
Attaching the Cuffs
If your sleeve is gathered onto the cuff, stitch the gathering stitches from one side of the cuff opening to the other. Gather the sleeve onto the cuff the way you would a ruffle. If the sleeve has pleats or tucks, the cuff should go on flat against the sleeve.
If your sleeves have plackets, the cuffs should fit exactly on each edge of the finished opening. Another type of opening might require the cuff to extend beyond the edge of one side of the opening to create an overlap. Follow your guide to place the cuff correctly.
Generally, the cuff is sewn to the shirt with the right sides together. Then the seam allowance is tucked into the cuff. The underside of the cuff is blind-stitched to the seam. Often cuffs are topstitched to finish them.