If you are buying a new CAI from Rob ( Needswings ), then it comes with instructions.
If you are buying a used one, please be aware there are 2 different versions.
The original one has a bung in the 3" tube for the IAT/MAF sensor and a 90 deg bend that connects to a 90 deg rubber/silicon sleeve that then connects to the throttle body.
The later version uses a plastic piece to hold the sensor between the TB and the 3 " tube.
I have the original one - take a look at my gallery.
You can see the new one on the Needswings site.
The only difficult part of the install is removing the plastic piece by the radiator that the old engine cover plastic pipe connected to ......

NOTE: bite the bullet and replace the 68mm TB with a 74mm one at the same time like I did, then get the tune.

( you can also get a C320 engine cover to hide the naked engine, if that bothers you )