Sometimes you have the perfect thing to wear, but it doesn't quite fit perfectly. Don't give up. You can take your too-big jacket to a tailor, or you can tailor it yourself. Don't try to take in fur, leather or any material that's too thick for your sewing machine, and don't try to alter a garment that's 5 sizes too big for you unless you're an experienced tailor. But lightweight jackets can be taken in easily if the correction isn't extreme.
Put on the jacket inside-out and fasten it. Be sure to wear something you think you'd normally wear underneath it. Standing in front of the mirror, pin the side seams until the jacket is roughly the size you want. You can also have a partner do this if you find it challenging.
Take off the jacket and look at the seams. If you've pinned more than 2.5 cm (1 inch) on each side, move on to Step 3. If you pinned 2.5 cm (1 inch) or less on each side, you can just take in those two side seams. Double check to make sure the pins are in the right place and the jacket is now the right size. Sew where the pins are, tapering gently up toward the armhole, and move on to Step 4.
Pin two darts into the back of the jacket at the waistline. Position them on each side of the back, dividing the back evenly into four sections. Each dart can take up to 2.5 to 5 cm (1 to 2 inches) of extra material at the waistline. Put on the jacket inside-out. Take up the remaining slack in the side seams and pin. Sew the darts and side seams, taking care to taper curves gently.
Press the seams flat. Try on your new jacket.
Unless the excess fabric you've taken in is very bulky and bothersome, do not cut it; it will fray and be unsightly. If you need to cut it, use pinking shears or sew a line of stitching next to the raw edge of the fabric to prevent it from fraying.
If the jacket is lined, you may want to take in the lining separately from the shell.