Idler

You will need the following parts:

Assemble the idler bearings as shown.
The screw head and nut can then be seated into the idler. Make sure it is seated fully in, and the bearings are level with the idler printed part.

Idler bracket

You will need the following parts:

Push the M3x16mm cap head screws through the idler bracket, and push each M3 nut it its recess in the idler printed part.
Screw the idler up into the recess in the idler bracket. This will later be used to tension the drive belts, so may be left loose at this stage.

Completed assembly

You will need the following parts:

Push the M3 nuts into their sockets in the idler bracket.
Fit the button head screws, leaving them protruding at least 3mm.

Repeat

Now repeat the above steps until you have three assemblies.