Turn your jacket inside out. You should be able to see the seams of the jacket, including the sleeves.
Measure the sleeves and write down the measurements. Do the same for the back panel and two front panels of the jacket.
Using a pencil (or chalk, for dark liners) and the measuring tape, draw out the measurement for each jacket part on the liner fabric. Start with the back panel.
Use the scissors to cut out each liner component. Leave a half-inch of fabric around the edge for sewing.
Pin the liner pieces to the inside of the jacket using the straight pins. Line them up carefully, ensuring that the liner is flat and smooth throughout.
Use the needle and thread to sew each liner component to the inside of the jacket. Match up the pre-existing seams, cutting away excess fabric when needed. Rock your new, warmer jacket.
Use a thin fabric as a liner. Thick fabrics tend to bunch up and make you look bulky.
Measure and sew carefully. A few extra inches will cause bunching or unwanted folds.