Choosing a Location
Overhanging branches from trees may seem the logical location because they offer some shelter and block the wind. Starting a fire beneath snow-covered branches, however, is an aggravating and often pointless task. Heat from the flames will quickly melt the snow above and it will fall, smothering your heat source. Instead, find a natural overhang that will protect the flame, but which does not have precarious build-up of snow atop it. A rock overhang works well, but if none are around you should prop up a piece of bark or rock slab on an angle. A poncho or tent tarp will work well, too.
Building a Base
A fire built on bare snow will quickly melt the ground and extinguish the fire. It is, therefore, necessary to build a base, on which you can build the fire and keep it dry and protected. When cold, green, wrist-thick tree limbs will snap easily and can be used to build a flat surface that will not burn too quickly. According to the U.S. Army Survival Manual, collecting several of these limbs and arranging them parallel with one another along the ground will serve as a suitable base for your fire. For a longer-lasting base, stack several layers of limbs.
If you are below the tree line, fuel will be abundant, even in winter. Collect dried moss or shrub material that may still be available on tree trunks or sticking above the snow. These materials burn quickly and light easily, and are good for starting the fire. Next collect small twigs and dry branches. Then begin collecting larger branches that will burn longer. Several types of trees offer different advantages for survival fires. For instance, birch trees burn hot and fast, making a fire easier to keep going and more effective for preventing hypothermia. They grow close to water at lower elevations. Pine trees, and especially pine needles, produce a lot of smoke, which can be used for signaling in an emergency situation. If fuel or oil from a wrecked or broken down vehicle is available, use it to start a fire in the absence of alternative tinder. A small amount of fuel can help start an already prepared pile of tinder, making the fuel oil last longer.