Place 1x Z upper [PP-GP0525] inside the 747 Z upper jig with the magnet on the top.
Then take 2x T-slot extrusions [HD-EX0098] and slide them into the Z upper.
Following [reference#1] slide 6x T-slot nuts [HD-NT0053] into the extrusions.
Place the other Z upper inside the 747 Z upper jig making sure the extrusions are fully slide in
Using 4x BHCS M5x20 [HD-BT0272] secure the extrusions to the Z uppers.
Note: Start the screws with a screw driver, then once all four are started use the drill to tighten them.
Once the extrusions are secured flip the assembly over and using 4x BHCS M5x10 [HD-BT0073] and the four T-slot nuts inside the extrusions secure the Z uppers.
Note: To align the T-slot nuts use a tweezer or something that is not magnetized