You have two Mounting Rails – one for the left side, which should be marked with an “L” and one for the right side, which should have an “R” on it. The one end with one thread with a plastic tip on it is the back, which should go in first. The other end with two threads is the front, which should be facing you after you mounted the Mounting Rails.
Take one Mounting Rail (don’t forget to use the correct one for whichever side you are starting with) and loosen the hex nut in the middle of the Mounting Rails to later extend it while mounting it.
Now guide in the Mounting Rails with the black plastic tip first. This tip should be on the upper half of the rack unit. Note: Be careful that both Mounting Rails are on the same level on both ends. Otherwise, your hardware would be skewed.
The other end of the Mounting Rail (with the two threads) should be on the same rack unit and placed in the 19” rail facing you.
In case your Mounting Rails are too short, loosen the hex nut if you haven’t yet, and extend the Mounting Rail by pulling on one end.
Now guide the other end into the threaded holes as described in step 4, fasten the hex nut of the Mounting Rail, and also fasten the Mounting Rail on the 19” rail with the hex nuts.
Repeat the process on the other side of your 19” rail.