Brown relates when he and his friend, under the tutelage of his adopted grandfather, an Apache elder named Stalking Wolf, spent the night in the wild and their efforts at building a lean-to. The first night their shelter was just a stick collection, and they froze. Stalking Wolf admonished them to heed the wisdom of the squirrels. So the boys, after seeing squirrels carrying leaves, threw more leaves on their shelter. By the next morning, the leaves were gone and they froze again. Then they examined the squirrels' nest and noticed the intricate arrangements of leaves and twigs, so they worked these then and thought they had success. It rained, but they were warm. Stalking Wolf, however, continued to advise them to heed the squirrels, and finally the boys caught on. They rebuild their shelter, throwing out the wet leaves and in addition to rebuilding the networked twigs and branches, they mimicked the exterior of the squirrel's nest with a bulky slope, strong enough to repel water. That night the wet leaves froze, but the boys didn't, and finally earned Stalking Wolf's approval.
So, begin with the basics, and heed the wisdom of the squirrels. First, you will need large branches to carry the weight of the total structure. Arrange the branches so they will interlock and support each other. Buttress these branches with support in the ground so they won't fall. Once the large support branches are in place, begin building the walls with smaller limbs and branches. If you have an axe or saw, you can cut leafy boughs and position them as walls. Interlock these boughs with your support limbs and each other.
Add leaves, pine needles, and other insulation to the shelter. Pile leaves and other material into the shelter, and continue to interlock it with the previous building material. Fashion the exterior of the shelter into a rounded slope, so water will run down the sides of your shelter, not on you inside of it.
Don't neglect the insides of your shelter either! Bring in leaves and other material to bolster your comfort from the inside as well as the out.
A final word of advice: make the shelter small. The smaller the area you have, the easier it will be to construct and keep warm in. Remember, you're building a temporary shelter, not your permanent estate.