Lift your undermount toolbox up under the tray, behind the guard. Using M8x25 bolts with a washer each side, fasten the toolbox to the tray using a nyloc.

Ensure your toolbox is in line with the tray and not askew.

Keep the gap between the toolbox & bumper and toolbox & guard equal.

Once your toolbox is in the correct position fasten the bolts that mount the toolbox to the tray and the bolts that fasten the mounts to the toolbox. 

If you have front undermount toolboxes, the same process is followed as above.

Make sure your front toolbox is in line with the front of the tray/headboard

Once your toolbox is in position, fasten all bolts.

Repeat steps for the toolbox on the other side. 

Your front undermount toolbox is now complete