How to Build a Lean To
To build a lean-to, start by finding a sturdy branch that's long enough to stretch between 2 trees. Then, tie 1 end of the branch to each tree using twine or rope. Next, lean thinner branches against one side of the sturdy branch to create a wall. Additionally, cover the branches with a layer of leaves to insulate your shelter, and place small twigs on top to hold the leaves in place. Alternatively, pin up a tarpaulin for greater insulation and protection from bugs.
Check what the direction the wind is blowing. You do not want to build a lean-to that's leaning in the direction of wind. Even if the wind is not blowing fast now, it could pick up later and topple your lean-to over. The easiest way to check the wind's direction is to simply try to feel it. Close your eyes and turn your face until you can feel the wind blowing on you. If you can't feel the wind, look for signs. Check which way flags are blowing or look at the direction water flows. Look for two large trees. You will need to set up your lean-to between two large trees. These are used to support the large branch that makes up the backbone of the lean-to. You want the trees to be relatively close together. You will need to find a branch long enough to stretch between the two trees. Make sure the trees are sturdy. You want trees that can support a small amount of weight. Find a large, sturdy branch. This branch should be long enough to fit between the two trees you located. You should also make sure it's sturdy. This branch is the backbone of your lean-to. It will support all the other branches that make up the structure. You want a somewhat log-like branch. It should be thicker than the average branch you find in the forest. You may have to search awhile before you find the right branch. It can be hard to find a branch long and thick enough for your lean-to. Gather leaves and smaller branches. The smaller branches should be thinner than the backbone branch, but make sure they're long. When your lean-to is in place, the smaller branches will form the side of the lean-to, so they should be able to stretch from the backbone branch to the ground. You will also need bunches of old leaves to insulate the lean-to. Gather some tiny twigs and branches to place over the leaves as well. This will hold them in place. Gather a lot of branches, all in roughly the same length. You can break branches as needed to get them all the same length. The branches will need to be stacked close together to form a solid wall free of holes and cracks. Tie the backbone in place. You will need rope or twine here. You want to tie one end of the backbone branch to each tree. Make sure you tie the rope tightly. You want the backbone as secure as possible. Place one end of the branch against the tree at about the height of your shoulder. You can use your shoulder to support the branch as you tie. Wrap your twine around the end of backbone branch. Then, bend the twine in a 90 degree angle. Wrap the twine around the tree. Bend the twine down 90 degrees again and wrap it around the branch again. Bend it 90 degrees again, and wrap it around the tree again. You're essentially forming a square with the twine that wraps around the tree and branch. Repeat this process several times until the backbone branch is securely in place. Repeat with the other end of the branch. When you're done, the backbone branch should be forming a straight line between the two trees. Line up the smaller branches closely together. You will want to lean the smaller branches against the backbone branch. The branches should form roughly a 45 degree angle with the ground. Continue leaning the smaller branches until a solid wall is formed. Your lean-to should be beginning to resemble a small shelter. It's vital to lean the branches very close together. Any gaps in the branches could let in cold air. Make sure there are very few gaps or holes between branches. Place the leaves over the branches. Take your leaves and place them over the small branches. You want to have a solid coating of leaves covering the side of the lean-to. The leaves will insulate the shelter and help keep it warm. If possible, wear gloves during this process. When you finish, lay the smaller twigs and branches over the leaves. This will hold them in place. Use your lean-to. You can now use your lean-to as shelter. You can sleep in your lean-to, as long as you make sure to lay padding down on the ground, use a sleeping bag, and wear many light layers of clothing. However, it's more common to use a lean-to as a temporary shelter to use when it's raining or cold outside. Check your leaves for bugs. You want to avoid bug bites when camping. Before adding your leaves to the shelter, sift through them to check for bugs. If you have a rake, raking through the leaves first may help remove any bugs. Carry insect repellent that contains DEET when camping. This can help prevent mosquito bites. Carry a tarp when camping. While leaves can work on a lean-to, it may be safer to pin a tarp up for insulation. You are less likely to be exposed to bugs this way, and the insulation may be greater. Try to remember to bring a trap each time you go camping. Make sure you have adequate bedding. Hypothermia is a real concern when camping, especially if you're using a shelter like a lean-to. If you live in a cold area especially, make sure to bring adequate bedding and clothing. Always bring a plastic ground cloth to lay down under your lean-to. Wear many layers of lightweight clothing. 
Check what the direction the wind is blowing. Look for two large trees. Find a large, sturdy branch. Gather leaves and smaller branches. Tie the backbone in place. Line up the smaller branches closely together. Place the leaves over the branches. Use your lean-to. Check your leaves for bugs. Carry a tarp when camping. Make sure you have adequate bedding. 