A lean-to is an overnight shelter to build in a wooded or open area that has trees growing close to one another. You will need to carry with you, 25 feet of light line or rope and an 8 x 10 foot plastic tarp or drop cloth, equipped with tie lines or grommets.
Find two trees about 3 to 4 feet apart to serve as upright supports. Then find three straight, strong, sticks for the framework, two 7 or 8 feet long, and the other one about 1 foot longer than the distance between the two trees. Lash the short pole to the windward side of the two trees 2 feet from the ground if the shelter is just for sleeping; 3 or 4 feet if you plan to sit up and cook.
Rest the ends of the long poles on this crossbar so that the lean-to's open side will face leeward (downward), and lash them in place. Stretch the tarp over this framework and lash it to the crossbar. Fashion pegs from sticks to hold down the edges; then pile dirt to seal out the wind.
In case of an emergency, make a lean-to that is almost as weatherproof by covering the framework with slender branches an evergreen tree with a low branch, and lean evergreen boughs against it. In the winter time, pack snow over the boughs for insulation.