It's an advantage to attach as many structures as possible on a module so they don't have to be placed on the module when setting up. I use a couple of different methods and here is one of them.
On a structure with a solid base and that is being placed on a flat, solid module surface I drill and tap holes through the base of the structure before attaching the roof. I then screw in long screws matching the size of the tapped holes. In this case I used 4-40 screws.
When the holes are in the structure base but before the screws are installed, I place the structure in it's spot on the module and mark where the holes line up on the module base. Then I drill holes through the module base slightly larger than the screws.