This is the basic motor and coupler assembly for each axis. Starting with the stepper mounting plate, the bronze bushing is placed into the center 5/8" hole. I had to tap these in with a hammer which worked fine. The shaft collar goes on the ACME screw next followed by the 1/2" bore oldham coupler. These parts clamp on with the pinch screw. The oldham coupler is actually comprised of three separate parts, the 1/2" bore coupler for the ACME screw, the center disc, and the 1/4" bore side for the stepper. I decided to use oldham couplers because they have zero backlash and can handle higher misalignment. You need this because its going to be nearly impossible to get the ACME screw and the stepper motor shaft perfectly aligned. If you use a rigid coupler and the shafts are not aligned well you'll be putting unneeded stress on your stepper motor bearings and causing friction in your system which leads to parts wearing out much quicker. The oldham couplers are a little pricey at $30 for all three parts but they will save your steppers and ACME screws.
This is a video from Ruland
and has a great description of the the Oldham couplers. The couplers I have listed from Mcmaster are made by Ruland.