if you haven't found it already, it's very simple. put the disc in (but not being read, just sitting there on "my xbox" screen), and hit the "Y" button (it gives a prompt for this, noting it's for "details". At this point, one of your options is to "install disc to hard drive".

You have to do this for both discs, so you run the process twice and you're all set. Keep in mind, you'll still need the disc in the drive to play (drm), but it won't be reading off the disc itself.