This post in the Mods forum gives full instructions for installing mods and for backing up the original files.

One thing to beware of, though is that there are a lot of mods available, made by several different people, so they might not look right if you install two different mods. Take care to read the instruction given by the mod posters to see what other mods they go with, although if you post a request in the Mod Comments forum linked by vfmacfan someone will likely be able to help by creating a special map for you.