Most computer system use a standard drive cage to install a hard drive but some newer cases use a form of tray or rails. Here are the instructions for the two most common methods:
Drive Cage: Simply slide the drive into the cage so that the mouting holes on the drive line up with the holes in the drive cage. Fasten the drive to the cage with screws.
Tray or Rails: Remove the tray or rails from the system and align the tray or rails to match the mounting holes on the drive. Fasten the drive to the tray or rails using screws. Once the draive is affixed, slide the tray or drive into the appropriate slot until it is secure.