Most pattern instructions advise setting a sleeve into a garment with the side seams sewn. There’s another way to sew a sleeve which is helpful to know: sewing a sleeve flat.
This method is frequently used in factory sewing. Before you begin, make sure all the markings are correct on both the sleeve and the garment.
First, baste the sleeve head, and place your finger behind the machine foot. The fabric will gather up between your finger and the foot. This crimps the fabric simplifying the easing process.
Second, since neither the garment nor the sleeve have the sides sewn, it’s harder to accurately pin the pieces together. Instead, you align the two pieces at the seams and gently ease the sleeve as you sew it to the garment. Make sure to match all markings.
Once your sleeve is sewn, finish the seam allowance as you like then sew up the side seam!