There doesn't seem to be much documentation about this, but in fact it is quite an easy process.
Download the zip file from this page - http://custom.simplemachines.org/mods/index.php?mod=2210
Go to Admin > Package manager > download packages
Click on the upload a package link at the bottom
Upload the zip file you saved
Load the packages list, and click on install for the analytics package
It will show a box for an UA number - click on the get one link
Register with Google end enter the number the give you
You can check that it is working by logging onto your SMF site, and stay logged in. Open Google analytics in a new window, and click on the real time link. It should show at least one person on your site.