Step 1:
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.
Step 2:
Pull on one end to extend the Mounting Rails.
Step 3:
Put your Mounting Rail in place. When it’s long enough to reach the back and the front of the 19” rail, and the plastic tip and threads are in the threaded punched holes, fasten the hex nut.