Well, there are some tips you can just follow. First, you can apply a ruler, so that you can measure the overall width at the widest point. In common, it should be the temple of an old pair of glasses. Of course, you should lay your glasses on a table with the arms open and lay a ruler across the arms with the millimetres nearest the front of your glasses to measure. In that way, the number can be accurate. And then, just remove the lenses from your old glasses by unscrewing the screws that hold the frame together. But on the other hand, you need to remember that you should not force them out with your thumbs or fingers in case of injury. Anyway, just have a try.