Gather the top and bottom pulley attachments. These are the two largest pieces. The top pulley attachment is smaller with just two holes for shafts, and the bottom pulley attachment is larger with the two holes for shafts and two for bearings.
Screw the pulley onto both the top and bottom attachments of all the stages you’re constructing. Note that one side of each pulley mount has a larger hole than the other, so the screw should slide through one side and be tapped into the other. Don’t tighten the screw all the way, doing so may cause the pulley to rub on the mount. The screw only acts as a shaft for the pulley to ride on, not to fasten pieces together.
On just the bottom mounts, place bearings into each of the mounts, and screw them together with an M3 bolt, around 12mm works best. Note that 3D printed plastic can not withstand large amounts of force, so don’t over tighten the screw. If using a drill to screw it in, don’t rely on the drill’s torque breaking point, as even the weakest setting on most drills still strips the screws.
Additionally, install the belt mounting plate onto the ends of both the top and bottom pulley attachment. Use longer screw, around 25mm, and leave a gap of 5-10mm between the plate and the mount.
Finally, attach the shafts to just the bottom mount, once again using ~12mm M3 screws.