Choose a heavy duty pallet with 3/4-inch thick slats, such as those used for cases of meat and other heavy items. Strip the slats from one side with a hammer and pry-bar, leaving the other intact for the platform.
Use a belt sander to sand down the faces and edges of the slats you removed and the top and edges of the platform face of the pallet. Add nails as needed to firm up the platform side. Cut one slat to fit along the open front edge of the platform and screw it to the ends of the 2-to-4-inch thick runners under the pallet.
Cut the removed slats in half and screw them around the edge of the pallet on three sides with three 1 5/8-inch treated deck screw in each board, leaving 3-inch gaps between each slat. Save three full-length pieces and screw them to the top edge of the cut slats for a rail. Check the railing for rough spots and belt sand as needed.
Find a tree with a trunk that spreads at least in three directions and has a wide enough area to set your pallet into. Add 2-by-4 treated lumber braces down to the ground to level the platform and tie ropes through the slats of the platform around the runners underneath and around the tree trunks to hold the pallet in place.
Build a ladder from well sanded 2-by-4. Cut the rails to fit from the ground to the platform and enough 12-inch wide pieces to make a step every 10 inches. Place the rails on edge and set the steps, wide face down, on top with the ends flush with the outside edges of the rails. Bore two 1/8-inch pilot holes in each end of each step and attach them with 3-inch treated deck screws.