The first step is to measure the tram error, here’s a good method to do that: How to square the Bridgeport head with the table - YouTube
Note that you will need an articulating indicator mount that can be inserted into a collet so that you can manually turn the spindle shaft and sweep a circle on the table.
Set the indicator arm such that the tip is sweeping a 4" circle. Zero the indicator in the 12 o’clock position. Then rotate the spindle until the indicator is at the 6 o’clock position. If the indicator, for example, were to read +.005 That means the that the spindle is nodding forward. In order to eliminate the nod error, you would insert .005 of shim between the spindle housing and the Z slide plate at the bottom. If the indicator reads -.005, you would need to add .005 shims at the top.
After the shims are installed, you of course will need to check at 12 o’clock and 6 o’clock again to make sure the result is acceptable. Add/remove shims as needed.
Tilt tram is easier to adjust and should be done only after Nod tram is completed. The first step is to break loose and then snug down the four screws that secure the spindle housing. The screws should not be so tight that the spindle assembly cant be slightly rotated about the surface of the Z slide. Load the indicator back into the spindle and set it up to sweep a 4" circle. Zero the indicator at the 9 o’clock position and then rotate it to the 3 o’clock position. Bump the spindle as needed until the two measurement are as close to the same as possible. Then tighten the bolts.