The 2 sub assemblies shown last time mated together and fitted with some of the 3d printed parts.

On a simple set of loco frames such as an 0-6-0 loco where the sides are mirrors of each other you can easily check everything is square by popping them on a bit of glass and see if they rock.  If they don’t you are all good.  On a larger set of frames thats made up of sub assemblies, not all of which are the same height you need another way and the easiest Ive found is to use a few chunky bits of evergreen strip and see if everything looks parallel when viewed end on. Things can then be tweaked and when reasonably happy the heights of the ends measured from the surface to check each side matches. The above image shows my start point.

Moving on the bogie is basically 3 sub frames.  The finished bogie is shown on the right.

The motion bracket and cylinders. And assembled onto the frames.

Once in place the motion bracket and cylinders become one unit that is held to the frames by 8 small bolts.