Once the coat body and sleeves are assembled, it is time to set the sleeves into the body. This may seem obvious, but try to put the right sleeve on the right side of the body. I have sewn the right sleeve to the left side . . . more than once. There should be matching notches in the armscye of the body front and back pieces. Match the sleeve front and rear seams to the notches. You may need to ease the rest of the sleeve fabric to make it fit. It will. By all means, avoid any gathers or tucks. The armhole seam should end up smooth, with no puckering. However, do not press the shoulder seam open at this point! That comes later.