I have Photoshop, so it's all done in layers - though you can use any art program that supports layers.
I took the forest tree and cut off the shadow and the trunk. That became a base (layer 1) on which I layered the house.
I made the little house, which was a lot of erasing, copy-pasting and a few small retouches (layer 2). The door was made from one of the windows in the forest town set. The skylights are just tilted regular windows from the same set (just keep moving up sections of the pixels to match the angle).
I put the house on the topiary. It looked kind of flimsy, so I went back to layer 1 and added some volume (more copy pasting).
Then I went back to the house layer and added an elongated trunk from the big tree (layer 3). Besides copy-pasting, I smoothed out some pixels so it didn't look choppy. I added a ladder on top of that.
Lastly, I used the lasso tool which allows you to select various shapes and cut out some pieces of the topiary (layer 4). They went on a layer above the house, so they're obscuring the house and ladder a bit.
Hit save, uploaded and posted here.