Ensure the machine is powered off before proceeding.
The disks are mounted into the draws at the front.
Press the red button on the right of the draw.
By default 0 will be the OS disk, 1 will be empty, 2 will be empty and 3 will be empty.
The arm will open, pull this to pull the draw out.
Remove the empty tray and fit the hard disk, remember to put the power and SATA connector end at the opposite side to the draw arm.
Repeat for all hard disks.
Plug the machine in and turn it on.