I made a treeblock treehouse for dd for christmas.
I couldn't find specific instructions, but I liked the look of this kind
It's just made from tree branches-for the upright supports, then I used 1" pine for the floors-- cut curvy with the jigsaw, edges rounded with the router, sanded and finished with walnut oil.
To connect the branches to the floor, I used wooden dowels--drilled holes in the centers of the branches, then drilled holes at various places in the floor. I don't have a band saw, but I think that would help to cut the larger diameter branches/logs more evenly. I had precut all of my branches, but when I was assembling the house I found I hadn't accounted for the different angles of top/bottom so I needed to recut some.
I sized our treehouse to fit the 5" Kathe Kruse dollhouse dolls--so I think it ended up being bigger than the ones peopled by the tinier treehouse people--or the little tomtens. I want us to work on some wood furniture as a spring project.
Both dd and ds have been enjoying the treehouse.
+the best part is that the branches were found wood, so I only had to buy the pine board ($15) and the dowels ($5).