The following is a list of steps I used in building a lift for the treefort I built last summer. And yes it was my son's request or wish. This was a work in progress type of endeavor as the plan kept changing as I kept getting further along.
Step 1: Step 1
Secure a 2x6 pressure treated beam to a tree and 4x4 support post. Use a 6 in long 1/2 in diameter galvanized lag screw to mount the beam to the tree. Cut an oval hole in the beam to allow for tree movement.
Step 2: Step 2
Secure the beam to the 4x4 using wood brackets and 5/16 bolts. Brace the 4x4 with clamps at this point.
Step 3: Step 3
Secure the 4x4 to the deck using wood brackets and 1 5/8 in deck screws or 2 1/2 in deck screws depending on location. Also, use metal elbow clamps for extra support and to mount the brackets to the deck. Also, a wood block was attached to the 4x4 as a holder for the rope.
Step 4: Step 4
Add a support from the 4x4 to the beam. Use metal brackets and screws for mounting. Time to add the main pulley (rated 440 lbs.) to the support beam. Use two angle clamps and 3/8 in bolts.
Step 5: Step 5
At this point you could be done and just add a bucket to the pulley. Of course that wasn't good enough. Construct a platform using 2x4s and deck planks. Next, add guides so the platform does not spin when you raise or lower the platform. The guides are 10 ft 1/2 galvanized pipes mounted to the deck railing. The platform is attached to the guides with 3/4 in pipe clamps.
Note: more pulleys increase the mechanical advantage. I purchased the ones shown above from The Home Depot, however, I have a double wheel system on order which should make it easier to lift little boys and such.