For "regular" sleeves, sew the jumpsuit body's side seam and sleeve seam closed in one continuous stitch, from the bottom of the pant leg to the bottom of the sleeve.
Press the seam allowances open.
Fold the sleeve's hem allowance upward and press.
Hand sew the hem allowance to the sleeve.
For vented sleeves, sew the jumpsuit body's side seam and sleeve seam closed in one continuous stitch, from the bottom of the pant leg to the upper edge of the sleeve vent, pivoting at the vent and reinforcing the seam several times at the top of the vent.
Clip through all layers diagonally from the top corner of the vent to the stitch line pivot.
Press seam allowances open.
Fold the front side of the sleeve under ¾" (wrong sides together) and press.
Cut a 3 ½" length of ¾" black hook and loop tape ("Velcro") - 3 ⅝" if you're feeling brave!
Sew the "Velcro" to the underside of the front sleeve bottom (the part you folded under) and the right side of the back sleeve bottom (the flat part).
Your finished sleeve vent should look something like this: