Hi, If you already own the vest and want to line it. You need to make a tissue paper pattern that copies the back, front left and front right sides. You can use wax paper or gift tissue paper or tracing paper. What ever you have in the house that is thin and some what see through....You need to mark on the pattern what ever important elements needed...darts? button holes etc. When you cut out the pattern you need to add an additional 1/4 inch. Now sew the lining back to the 2 sides and the shoulder seams. Try it on to make sure you have copied the vest size correctly. It should fit the same as the vest. I would use a light weight fabric that is somewhat smooth, so it is easy to get on an off. Fabric stores have inexpensive lining fabrics in all colors. Put the vest and the lining, Right Sides together. Sew around the outside edge of the vest. 1/4 inch seams. Leave a 3inch opening to turn it inside out. Slip stich this opening closed.Press, Tack down the shoulder seams so they stay matched. Turn the arm holes in 1/4 inch on both the vest and lining and hand invisible hem the armholes... Presto, you have a perfectly lined vest...for very little money and a little work on your part..Feel free to contact me if you have further questions. Best of luck. Have fun...Donna