While it might not really be necessary on an Ender 3 to fix the z-screws at the top, I got the flanged bearings anyway when purchasing a second lead screw (Amazon link), so I wanted to at least put them to use.
The bearing must be slid onto the lead screw first, then the plastic part can be mounted.
The adapter is attached to the extrusion using M3x10 countersunk screws and t-nuts.
I simply superglued some M5 nuts to the bottom of the bearing and used M5 cap head screws to attach it.
In order to align it properly, raise your x-gantry as far as you can, then fasten the M5 bolts (the holes in the mount are intentionally oversized to allow for slight inaccuracies elsewhere).
Finally, tighten the two grub screws on the bearing to lock it onto the lead screw.
Onshape Link: https://cad.onshape.com/documents/bbb4322bac733629ee2f0537/v/e32a1adf06346c990442e976/e/15c5ae4882678281e6da3333
The author marked this model as their own original creation.