I'm working on the staircase also. I dry fitted the staircase without railings into the house, that's fits in much easier than with the railings. I think I'll attach the railing after the basic stair structure is in place; putting it in with the railings attached is awfully difficult to put in place. Trying to install a completed staircase is a tight fit and possibly cause the railing to crack when trying to put the posts into those 2 slots in the 2nd floor opening.
We'll see how it goes.