I answered this type of question once but didn't find it in a search. It is dangerous and can be a knuckle/hand/face buster if you don't watch out. Questions first. Is this a single or double door? If double, are springs in middle and one still ok and if ok how many turns are put in it. Do you have a replacement available of the exact same size lenght, diameter of open hole, diameter of spring wire, and turns of wire per inch? If single, do you know how many turns were put in it. Some jobs are better left to the garage door companies that have the springs and do it for a living. They aren't that expensive although it can take a couple hours or more.