You should install the optical drive before the mainboard – doing it in the reverse order would be inconvenient. First, put the included faceplate on the drive:

It is fastened with two-sided scotch. First you remove the default faceplate:

Then you stick the new faceplate onto the drive tray:

Then you just put to drive down into the appropriate bay:

The faceplate is a little wider than the device itself, so it is firmly fixed after you insert it – there’s no need for further fastening.

You don’t have to dismantle the cage to install the drive – just unfasten the two top screws and you can easily insert it there.