A fallen tree shelter is great for a single person to stay in for a night or two, and is probably the simplest shelter to make. The best trees to use for this is something with thick foliage. Pine and fir work the best. One you find your tree, cut partially through the trunk to fall it, but leave it connected to the base so it stays at an angle.
Cut off just enough branches on the under side to make room for yourself. The smaller the area the warmer you will stay. Next trim the top, again as seen in the picture. Take some of your trimmings and lay on the ground under the tree to make your bed, with the rest build up your roof. Lay the branches so that the stalks are at the top and the soft ends are on the ground. the thicker you can build up the roof the better. Now crawl in and enjoy your warm new shelter!