Take 4 screws and mount the spacers to the hard drive and to the case. If they don't line up, then you probably measured wrong : )
Screw both sides and it should stay stationary. In this case, it sat perfectly on the metal space where a case fan would be mounted. If your case isn't similar to this, you may take 2 more spacers and put in some extra brackets.
That's it! If you need anymore space, just lower the brackets to screw into the slot lower and drill some more holes to add even more hard drives. If this guide helped at all, let us know!