Use a tree that is wide and sturdy with a thick trunk. It must have at least two to three thick low branches that grow horizontally to the ground, as well as branches in the middle of the tree that grow vertically. The key: Make sure that these branches can support the weight of a tree house platform.
Spend time circling the tree to pick the branches where the tree house platform can rest. Then, pick out the upper branches where the second tree house platform will sit. In between these levels, a ladder will be nailed to the trunk. Now, measure the height and width, the height between the levels, and the size of each platform. Envision where each tree house wall will go, than determine the size with a tape measure. Using these measurements, draw a design as accurately as possible on graph paper.
Build a ladder from the ground to the first platform. Use a handsaw to create steps, each about two-feet-long, and nail them to the front of the trunk. Measure boards for the width and length of the tree house. Lay each one across the branches to make the first story. Lay them as evenly as possible and make sure they fit together tightly.
Nail the floorboards to the tree from any angle, making sure that they are secured with at least four to six nails (two on one side, two on the other, two in the middle). Use as many boards as you desire, but make sure the platform is large enough for several people to sit comfortably.
Build a ladder from the first level to the second floor. This should extend past the second story. Remember: The second story is also the first floor's ceiling, so allow for at least six feet of clearance.
Climb the ladder and lay each board across the branches for the second story, just as you did for the first floor. Provide room between the trunk and the second-story platform where the ladder emerges, so people can climb onto the second story.
The second story does not have to be as large as the first, and it can be used more as a sitting area.
Create walls for each floor. Take a board that fits the width measurements. These will serve as railings, or walls, for safety. Nail these boards from tree branch to tree branch, around chest height. Use the saw to trim the boards if needed. Repeat this step with the second story of the tree house.
Add more boards as desired. Just make sure that the two stories of the tree house have railings or walls that will keep people safe.
Add furniture or blankets, such as plastic patio chairs, end tables or seat cushions, to add comfort to the tree house.