George,
First, use similar materials for platforms for either the abs or styrene handrails so you can bond them with Bondene solvent. If your base is a different material, use CA with an accelerator.
Handrail posts are 1-1/2 inches on center. If you make the platforms multiple lengths of this number you can end up with a post on each end. If this is not possible, use a piece of angle. Plastruct A2 or 3 works well.
I always glue the bottom kick plate of the handrail directly down to the platform and use a square or my eye to make sure it is perpendicular to the platform as the glue dries. You can also glue to the side of the platform to keep it squared up especially if you use a channel or beam to support the platform.
The styrene material is easier to work with since it is a little more ductile than the ABS. It works better for curved structures.