Can't find any decent links, but it's not too hard. First you need to know where Factorio is keeping the mods on your system.
For Windows: C:\Users\<user name>\AppData\Roaming\Factorio\mods\
For Linux: ~/.factorio/mods
For Mac OS X: ~/Library/Application Support/factorio/mods
Then you will need to download the code from the Github site I linked above. There is a green button labeled Code that has a Download Zip option when you click on it.
After you have it downloaded, extract the folder from inside the Zip and place it in the Factorio mod folder. Then rename it to Rampant.
You will want to keep an eye on this site for an official update to the mod. Once it arrives, just delete the Rampant folder from your mods folder and update Rampant from in-game like normal.