Most 3d printer start with building the entire frame. This printer starts with a base of 34 x 64 studs (19.2 x 51.2 cm). This is build in the middle of several base plates with a total size of 56 x 64 studs.
After building the base we determine the distance between the smooth rods for the Y axis. The Y-carriage places the LM8UU Bearing with a distance of 170mm. This gives 21.25 studs. The 1/4 stud is solved by an adjustable smooth roth connection (second image). The added fence gives the needed distance. Use 4 handle bricks for placing the smooth rods (third image). Total height of the holders is 4 bricks (the Y-carriage moves above the stepper motor). The rods are 50 cm each and are attached with loom bands (last 2 pictures).
The Motor is placed using technic bricks at the back of the machine. The pulley is attached to the motor. The bearing holder at the other side is adjustable. Use loom bands to tighten the toothed belt at the bearing side (4th image). Attach these looms after connecting the toothed belt to the LEGO-carriage.
The toothed belt is connected to a LEGO-carriage. The 2 blue connectors (5th image) fit in the LEGO technic piece below the metal y-carriage (first image).