If your jacket does not fit properly anymore, because you have lost weight, you may want to alter this garment rather than buying a new one. This can save you money in the long run and just requires a sewing machine and minimal sewing skills. After you have some experience sewing or altering your clothes, the task will be much easier.
Determine the measurements by using a measuring tape prior to making any alterations to the coat or jacket. Take measurements across the shoulder blades. You want the jacket to hug the shoulder, but it should not be so tight that it buckles. Look at the length to make sure it is adequate. Generally, the jacket length from the top edge of the collar to the lower hem should be equal distance from the lower hem to the floor. Make sure the sleeve hem ends near the wrist bone. Record the measurements on a piece of paper so you can look at them later. Turn the coat inside out and pull out the stitching in the seams by using a seam ripper if the garment is lined.
Stick pins in the fabric to mark where you should sew the hems. Use basting stitching in the seams to guide you where to sew the hems and so the fabric does not budge while you are sewing. To make basting stitches, loosen the thread tension on the sewing machine and place stitches at 1/4 inch intervals. Take out the pins and increase the thread tension on the sewing machine for the permanent stitches.
While you are sewing the seams, use the baste lines as a guide. Alter the seam at the shoulders,the base and sides of the jacket. Check that the lining ends 1/2 inch above the hem of the jacket if the garment is lined. Make sure the thread matches as closely to the jacket color as possible so the alterations blend in.
Put the jacket on and see how it looks in the mirror. If it does not fit properly, then you should make adjustments at this time. Cut off the seam allowances with scissors once you are done altering. Pull out the basting stitches with a seam ripper.
Seam ripper (optional)