Attach the POM leadscrew nut to the X axis mounts with 4x M3x14 socket head cap screws or button head screws, 8x M3 washers, 4x M3 lock washers and 4x M3 nyloc nuts. Do not screw them completely tight. You'll need to make some minor adjustments so you want to be able to move them back and forth.

Thread in your 8mm lead screws through the POM nut into the spider coupler. Do not tighten any screws yet.