A truss rod is installed for a couple of reasons, the first is when a neck has been thinned enough that the wood can no longer support the string tension being applied to it without bending and the second is usually during manufacture of a neck.
This walk through shows both scenarios
Time to repair approx 1 hr
We need to fit a truss rod in during the fabrication of a neck.
Marking the centre point of our wood.
Truss rod is 6.24mm wide so we will route using a 1/4 inch cutter (6.35mm)
Fit the router bit to a router table, and align the mark against the router bit.
Simply slide the neck along the fence and over the router bit