how do I make this staircase?

When I made mine, it was important to make sturdy 4 footed landings for the stair sections to attach to.

I am working on this same staircase right now (mine was a kit), and while the kit had a small support post, I think I am going to build a complete wall underneath it- maybe even make a little closet under the stairs.  I will probably make a mock-up using foamcore board first.

This was my FIRST staircase and had no idea except to measure my real stairs as a guide.