Step 6: Build the platform
Using 3" deck screws, attach the four 2 x 6's perpendicularly to one of the other 2 x 6's. Make sure they're spaced so that when you put it up in the tree, the perpendicular joists will miss the tree! With someone's help, put the contraption up in the tree, center it, and tie it down.
Screw the other 2 x 6 to the other end of the platform, and check that it is centered and square. For squareness, measure the diagonals and ensure they are the same.