Removing two screws on the back of the housing allows you to remove the back panel and slide out the drive tray. The drive try rests in the same track as the screws.
In the below left image we have the top of the drive tray, where the 2.5" hard drive will be attached. I immediately noticed the screw holes on the tray, which is a nice feature as it secures the drive to the tray. I have looked at similiar devices and found the drive to be held in place either by a tight fit, or only by the SATA connectors. Finally, there are two little LEDs by the hard drive SATA connectors. On the back of the tray, you will find the chips which will do the translating from SATA to USB.
Installing a hard drive is very simple, just as easy as installing a drive anywhere else. Slide the drive onto the connectors, and flip the entire tray over so you can secure the drive to the tray with four screws. Now simply slide the tray back into the housing, and re-attach the backplate. You now have a portable 2.5" hard drive.
Now that you have completed the installation of the drive into its enclosure, you can slide this assembly into the internal chassis.
As shown below, I have opted to install this into an available 3.5" drive bay. The enclosure slides easily into the chassis, and when it has been inserted all the way, the silver button pops out. To eject the enclosure, press the silver button and it will release the enclosure from the chassis. The below right image shows the two LED lights from the hard drive tray. These are very basic, with red indicating activity and green indicating power.