Most helpful was this one I found on a French blog, Les Mains Calmes, even though I don't speak more than a couple of words in French ^^
I used 3 mm Balsa wood for the sides and treads, and 2 mm for the risers.
I managed better than I anticipated, and I felt a little bit proud for trying, because I thought I'd had to buy it . First I glued half of the risers in place. Once I'd figured out how deep and wide etc it should be, it wasn't that hard.By the end of the day, I glued all the risers and half of the treads.
The next day I did the treads and sanded everything. For the photo it's in Draco's unfinished room(box) agains the right wall, but it will be placed in the entrance hall of the Hogsmeade house against the left wall.
Later I plan to make a handrail and bannisters on the right side to prevent Penny from falling down the stairs ^^ I'm fearless now ghe ghe