Step 4: Mount the main supports
Get a strip of light wood and nail one end to one of your trees at a height about 1 ft lower than you want the floor of your treehouse (to save a future concussion, it should also be at least 1 ft higher than your head!). Get it perfectly horizontal with the help of a level and nail the other end to the other tree. Drill 5/8" holes straight into the tree just above the strip of wood. Do the same on the other side of the trees, this time taking the extra precaution of first ensuring the new strip is not only horizontal but also level with the strip on the other side of the tree.
Now, take down the strips and measure the exact distance between the holes. Subtract this distance from 6' (not 8', unless you don't want the entrance platform), halve the remainder, and make a mark this distance away from one end of your 2 x 8. Drill a 3/4" hole in the middle of the board. Make another mark using the between-the-holes measurement . Now drill two 3/4" holes, each 1-2" either side of your mark and both in the middle of the board. Get a jigsaw and make two cuts between the holes to make a 2-4" long slot. Repeat for the other side of the tree. The slot allows the trees to move without tearing your treehouse apart - the more your trees move, the longer the slot ought to be (note that the slots I cut are only about 2" long, but these trees don't move perceptibly at the height of the treehouse, even in a strong wind. If your trees move appreciably, and/or if you're planning to build higher up, use a sliding beam support).
Now, screw your boards to the tree with a wrench. Use washers, and don't bolt hard against the tree. The space you're giving it to grow is the gap between the support and the tree. The longer you want your treehouse to last, the further you should perch the support away - and the more substantial your lag screws ought to be! I know my tree grew only by about 1/4"-1/2" in diameter over 4 years, but most trees grow faster than this. When I rebuilt the treehouse with 3/4" lag bolts, I gave it about 1/2" to grow on either side. I used 10" bolts for the trees with one bolt in them, and 8" bolts for the tree with two bolts in it.