Step 4: Mount the main supports
Now, take down the strips and measure the exact distance between the holes. Subtract this distance from 10', halve the remainder, and make a mark this distance away from one end of your 2 x 10. Drill a 5/8" hole in the middle of the board. Make another mark using the between-the-holes measurement . Now drill two 5/8" holes, each 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 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, not 4", 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! 3/4" or, ideally, 1" would be best.