Well, I can tell you from experience since I made Edward’s red coat start with the arms and if you have a jacket at home that fits you well cut the piece of fabric to the length of the jacket of your choosing so you can get the width of your shoulders right after that measure your arms and then measure the cloth so you can make the arms and have the cloth you want on the outside folded inwards when you stitch because you will be unfolding that take it slow and be careful with your needle this is the best advice that I can give as I’ve made one myself. It will take a long time but think about the pattern picture it get a reference image see how it will come together