that depends. Are you replacing the entire thing? I.E. the mirror and the casement. Or just the mirror. Replacing just the mirror is fairly simple. They have ready cut mirrors that are designed to be placed right over the existing mirror cracked or not. Or they have mirrors that are designed to be set in place of the broken mirror. Those you would have to do extra work and remove the broken parts as well, make sure it is good and clean before apply the mirror. Now when you apply the mirror they usually have these pathetic double sticky sticker things. You can use that, but urethane or a non acidic silicone would hold it better. But it has to be non acidic or it will eat through the silvering on the mirror.