A lean-to is a nice simple shelter to construct. You can lean this type of frame against almost any natural structure.
Look for a relatively dry, well-drained area. Check the wind direction to ensure that the back of your lean-to will be protecting you against the wind.
Gather a pile of strong limbs (poles). Look for smaller thinner branches you will use as well.
To construct the support for your shelter place two shaped poles upright into the ground. You can even use the trunk of a tree as one or both of your main poles if available. Place another pole between the two uprights creating a crossbar.
Use your additional poles to form the lean-to frame. Lay the poles at an angle from the ground to the top crossbar. Weave smaller, bendable branches in and out of these poles.
Cover your frame with whatever material is available. Start at the bottom working your way up filling the gaps with leaves, twigs, pine needles, etc. If you need additional protection, you can add sidewalls to your shelter using the same steps.
Place leaves, pine needles, or grass inside the shelter to make a comfortable sleeping or sitting mat.