Step 13: Assembly: Sleeves
Fold sleeves in half vertically, pin, and sew.
Mark the center (at fold).
Turn coat body inside out; align the sleeve seam with the side body seam, pin; find center of "inset" and pin "sleeve" center to "inset" center.
Follow the edges around the sleeve opening, pinning fabric together.
Sew... which can be kind of fiddly, so take your time, hand-sewing if it is less nerve-wracking.
Repeat with second sleeve.