What is a key step in constructing a garment's sleeve?

Constructing a sleeve involves several stages, but the key step that is unique and specific to sleeve construction is setting in the sleeve cap. The sleeve cap is the curved upper portion of the sleeve that must be eased into the armhole (armscye) of the bodice. This process requires careful alignment of notches, even distribution of ease, and precise stitching to ensure the sleeve hangs correctly and allows comfortable arm movement.

Setting in the sleeve cap involves:

1. Running an ease stitch (gathering stitch) along the cap between the notches to slightly reduce its length to match the armhole
2. Matching the sleeve's notches and seam points to corresponding points on the bodice armhole
3. Pinning the sleeve into the armhole with the right sides together
4. Distributing the ease evenly so there are no visible puckers or pleats
5. Stitching the sleeve to the bodice along the seamline

The other options describe general garment construction steps that are not specific to sleeve construction. Hemming the bottom is a finishing step for the garment's lower edge. Attaching the collar pertains to the neckline. Sewing the side seams joins the front and back panels of the bodice or skirt. None of these is a defining step of sleeve construction.