Ok, lets see if I can make this any clearer by posting visual aids(tee hee, aids). Im going to do everyone from top view because the only thing other views are used for in this care are adjusting the height of the stairs.
I probably made this way more complicated than it needs to be, but thats how I do it.
Ok, first you have your staircase. Look at that staircase, what a beautiful staircase!
Now you make a square box thats equal in width to your stairs:
Now clip it diagonally in half:
Now clip THAT in half(or just clip it at the bottom left corner) but keep both sides of the clip:
Copy and paste one of your stairs and rotate it 45 degrees and align it to the bottom left corner of your triangle(To line it up perfectly you WILL need to use vetex manipulation n the corners or resort to aligning it with a 1-unit grid size):
It makes it much easier if you use vertex manipulation to align the angled stair to a semi-high grid size(32 or 16) because it makes later constrcution very simple(in that you dont need to use the 1 unit grid, which will get very annoying after a while).
Now select the top triangle and use vertex manipulation to align the top corner with the bottom right corner of your angled stair:
Now that it looks hideous because the angled pause in the stairs completely juts out from the stairs. To fix it, move around the center verticies of the two triangles untill you think it looks right. This should be as simple as moving them a few units down and to the side:
There you have it. The top triangle is perfectly aligned with the angled stairs so making a compatable wall will be as easy and clipping a brush along the edge of the stairs:
Edit: Oops, you're pictures implied that you wanted to make a staircase. now this tutorial is useless. Whatever, now you know how to make an angled staircase.