⬢Take two of the BASE parts and insert the M3n nuts into the holes in both of them - see the picture (they are identical). If you can't push them in, insert a screw from the opposite side to pull them in.
⬢Flip one of the BASE parts and insert two bearings in it.
⬢Put the second BASE part on top of the bearings.
⬢Insert the M3x12 screw from the top and tighten it. Flip the base assembly over and do the same.
⬢Make sure both bearings can rotate freely. If not, release the screw(s) slightly.
⬢Repeat this step for the second base assembly.