Framing and building the house
Once the floor is in place and you are convinced everything is securely in place, you can begin the building of the house itself on top. Ideally, the treehouse should be entirely supported from the floor, with no contact with the rest of the tree. This prevents the build-up of stresses from parts of the tree moving in different directions. The supports should take care of all the tree movement. Occassionally this isn't practical, as will be discussed later.
The actual construction is similar to how you might make a ground based house or playhouse now that you have a solid wooden floor, so you can borrow techniques from other walks of architecture and design to make your design work. By now you should have a pretty clear idea of how the treehouse should look in the tree, preferably with a plan drawn up to guide the build and prepare materials.