Installation: It's easy, though you're going to be overwriting Spelunky's original assets, so you'll first want to back up your data folder. On Steam, you can find it in Steam > steamapps > common > Spelunky > Data. Just make a copy of that entire folder and store it somewhere safe. Download the mod from this forum page: just scroll down past the screenshots to the Dropbox link. (I don't want to link directly to his Dropbox). Extract the contents (three folders) into your data folder and replace the files when it asks. Then just start the game and say hello to Samus!

To get your old Spelunky back, you'll have to re-overwrite with your original Data folder contents.