Step 1 - make sure that all the holes in the printed parts will accept the bits that go into them. The 8mm holes that the smooth rod goes into on the motor and rod holders is often too tight. Clear the holes with the appropriate drill bit size. The smooth rod should be snug, NOT LOOSE. If you overdo it and it's loose, you'll have to make it snug again with tape or something until you can print replacement parts.
Get the X end without the motor mount and clip out the support piece that is in the way of the idler. Insert two linear bearings and put in a pair of 6-32 x 1" screws and nuts to secure the linear bearings.You may have to use a tiny nut to clear the M5 nut trap on one of the linear bearing holders (the one on the left below)
Build the belt tensioner in the same way that you built the one for the Y axis, using a 6-32 x 1" screw and nut plus a #8 nut and screw for the bearing axle, and a pair of 624 bearings:
Insert this into the X end that you prepared in the previous step, then secure it in place with a 6-32 x 1" screw and nut through the X end and the tensioner (I have the smooth rod in place here - it's probably a little early to do that).
Use the M5 threaded rod to press fit an M5 nut into the nut traps on both X ends:
SORRY I don't have good photos of the rest of this sequence.
During the next few steps, keep in mind that the motor mount end goes on the left and the linear bearings all face the same way, when assembled the linear bearings will all face the back of the printer.
Insert the two linear bearings into the motor-side X end and secure with 6-32 screws and nuts.
Insert the two longest smooth rods (470mm) into the X end that has the motor mount on it. Push until they hit a stop inside.
Put three linear bearings into the X carriage and secure them with three 6-32 x 1" screws and nuts. Slide the X carriage on with the two bearings on the bottom, one on top, bearings facing the same way as on the X end. Finally, slide the idler side X end onto the smooth rods.
Slide the short smooth rods (Z axis rods) into the linear bearings on the X ends.
Use three M3 x 10mm screws on each side to attach a motor to the Z axis motor/rod holders. I like to arrange the motor so that the wires face the center of the printer when assembled, just for neatness. NOTE that if your motor has a raised center section around the axle, you may have to trim the smooth rod holder a bit to get the motor to be flush.
Screw the M5 threaded rod into the nut in the X end. Spin it in about half way. Do this on both sides. Attach the M5 threaded rod to the Z drive motors using a bit of vinyl tubing on each side. Just let it be a friction fit for now.
Slide the top smooth rod bracket onto the top of the Z smooth rod a little bit, and insert the bottom of the smooth rods into the smooth rod socket on the motor end.
Lean the whole assembly up against the box frame with the motors sitting on the work surface. Start by attaching the LEFT side motor then the top support to the box frame with #6 x 3/4" wood screws. Adjust the position of the right side X end until the Z mounts on the right align properly with the wood frame, then secure that side with wood screws as well.
Use three M3 x 10mm screws to attach a stepper motor to the left X end. I like to have the wires facing up here for neatness. Put a pulley on the motor shaft so that it lines up with the opening in the X end - you will probably have to put the pulley on backwards - with the setscrew collar facing out.
If you can't get to the back of the X carriage through the opening in the box frame (IE if it's too high up), adjust the Z drive screws manually until you can get to it.
Attach the belt to the X carriage, around the motor pulleys, through the gap in the X carriage, through the idler pulleys and back to the X carriage, using the same zip tie technique as with the Y belt system.