Using 2x M3x16 socket head screws mount the bed mount to the Z MGN block. Make sure it's straight.

Insert the stepper shaft into the bottom of the leadscrew coupler and tighten the M3.

At this point you should be able to move the leadscrew and the POM TR8 nut in the hole for the bed mount. Move the Z bed mount down by twisting the lead screw until the bottom touches the coupler.

Center the POM TR8 nut and tighten down the M3x12 screws. You'll want to use a pair of pliers are a 5.5mm socket/wrench to hold the NYLOC nut.