If it were me, I would take off the sleeves to make the alteration easier. Once the sleeves are off, put the jacket on wrongside out. It might be best to have a friend help you with this next part. While wearing the jacket, starting pinning along the side seams to get the jacket more fitted. Don’t make it too tight, leave yourself some ease, so you’ll still be able to move around and raise your arms. Mark on the fabric where the pins are and do a quick basting stitch along the marks. Try the jacket on to see if you need to make adjustments. Next do the same with the sleeves pinning at the underarm seam. Once you finish with both, pin the sleeve back to the jacket to make sure the armcycle of the sleeve still fits pretty well back in the armhole. When adjustments are done, do a permanent stitch at the seams, cut off the excess and sew sleeves back on the jacket. Hope this helps and good luck!