A cap sleeve is a sleeve that sits close to the shoulder, fitting the curve of the shoulder. This type of sleeve is very short, but it is more modest than a sleeveless dress or shirt. It can make an outfit more fancy or it can increase the formality of your clothing. Cutting a quick cap sleeve and sewing it to your dress or shirt is an excellent way to get a brand new look for an old outfit.
Measure around the top of the armscye of the dress or shirt you want to modify. Measure from a point level with your armpit, over the top of the armscye and down to a point level with your armpit on the other side.
Measure around your upper arm where the sleeve will fall, and add 2 inches.
Cut an elongated half circle using the armscye measurement for the curve and the arm measurement as the straight edge.
Hem the straight edge by folding it up twice and stitching it flat.
Turn the dress or shirt inside out
Pin the curve to your armscye, pattern side of the fabric facing outwards.
Sew the sleeve into the armscye.
Repeat the process for the other sleeve.