I learned this excellent technique from couturier Jon Moore for setting in sleeves. If the sleeve cap is full or if the fabric is difficult to ease, run two gathering threads around the sleeve cap using a machine stitch length of 3.5mm. Pull up the gathering threads slightly. Working on a ham with the sleeve lifted up, use the tip of the iron to shrink out fullness in the sleeve cap. The tip of the iron should not go much further than the stitching line into the sleeve. If the fabric tolerates steam, use a bit of steam to help shrink out the fullness. Once complete, the sleeve cap should have some shape to it, making it easier to insert into the armsyce.
Working on a dress stand, pin the notches of the sleeve and garment together at the underarm. Bring the notch at the top of the sleeve cap up to the shoulder point. The seam allowances of the sleeve should be rolled to the inside. Once these two notches have been pinned, start pinning the sleeve into place, keeping an eye on the grainline and how the sleeve is hanging. The sleeve should hang straight. You can carefully try the garment on at this point to check the hang of the sleeve and the distribution of ease. If you are pleased with the look of the sleeve, transfer the pins to the inside and baste the sleeve and the garment together. Machine-stitch the seam, or if you wish to follow the couture technique, use a backstitch technique to hand sew the sleeve in place.