To attach the 4 x 6 ledger board to tree , first make sure it will be level. Stretch the board from the tree fort to the tree, level the board and make a mark with a hammer and prybar on the tree where the top of the ledger board needs to go. Pre-drill holes in the tree. Attach the board to the tree using 10" galvanized lag screws.
Lift the 4 x 6 bridge spans into place, and sue a router or belt sander to smooth any rough edges on the lumber.
Secure the beams to the tree fort with carriage bolts.
Secure the ladder to the bridge with lag screws. Attach the rope rail from the tree fort to the tree for extra security.