Lag two sets of beams to the tree using a 3/4 diameter x 10" lag screw. The top set of beams will be the floor height of your treehouse. Try to to drill the pilot holes as straight and level as possible. Next install the perimeter beams to the top set o beams and square them up using the 3,4,5 rule. Once the frame is square install your floor joist.
Step 2: Level The Platform
Level your platform by getting the floor somewhat level then mount a long 2x4 on one corner and go to the next corner and level that beam and install another 2x4. The longer the level the better. Now one of the four perimeter beams is level and continue to work your way around all the sides, temporarily installing the 2x4's.
Step 3: Install The Knee Braces
Once the platform is level you are ready to install the permanent braces that will support the treehouse. I used two 2x6's screwed together and cut at 45 degree angles. You can pick up some "hold down" brackets to mount to the tree with another 3/4" lag.
Step 4: Install The Decking
This is the best part of the project because you will be able to stand on the platform when this step is complete. To install the deck boards start at one end and let the boards hang over the edge long ways so you can make a clean cut with a circular saw at the end. Use a carpenters pencil for spacing your boards. When you get to the tree, use a compass with a pencil spaced about two inches out so to give room for the tree to grow and move in the wind.