A lean-to is one of the most simple shelters, which is what makes it so popular for camping and emergency conditions. Their primary purpose is to keep out rain, wind and other elements of nature, while possibly trapping in some residual body warmth. In a pinch, you can build a wood lean-to shelter out of found materials such as tree branches, vines and leaves. A tarp or ponchos could also be used as a covering. A more permanent structure can be made using logs and lumber planks.
Find a location. In the wild, choosing a good spot for a shelter can be a crucial part of its success or failure. If rain is a threat, don't build in a low area where water is likely to run and gather. Likewise, find a place protected from wind if that is a concern in the given situation. An ideal location will have flat ground and some trees for shelter and material.
Build a support. The premise of the lean-to is that, in contrast to a four-walled shelter, the roof simply rests on a single support and extends to the ground. The shelter underneath thus takes on the shape of the leaning roof. A natural object such as a fallen tree or a notch between branches can be used, or else a support must be built. This can be done by using two large branches or planks and anchoring them vertically into the ground. Fasten each end of a cross-tie to the verticals with twine or vine. Use the knife to cut appropriate lengths.
Lean the roof frame on the support. Whatever the roofing material is going to be, it needs a frame to sit on. This is usually the same material as the support, whether branches, logs or lumber. Lay the individual lengths perpendicular to the cross-tie, with one end touching the ground and the other resting on the support. If there is time and ample material, use twine to fasten the roof frame to the cross-tie.
Apply the roof. The roofing material will determine its degree of water-tightness. Leaves will be the least secure, but when lain in multiple intersecting layers can be quite effective. Otherwise, a tarp or poncho can be laid over the roof frame. If lumber is used, provided there is enough to cover the frame, a very tight roof can be made by situating the planks in a seamless cover.