Find the Right Spot and Dig
The ideal place to dig a snow cave is relatively flat and already has around 5 feet of snow. Forest area is best because tree growth ensures that the spot isn't an avalanche path. According to the Northwest Weather and Avalanche Center, two snow-shoers in the Cascades were killed in 2001 after setting up an emergency camp in an avalanche shoot. The accident could have easily been avoided if they had chosen a more suitable spot.
Forested areas are also a great choice because the trees will create snow drifts that are easier to dig into. Always start by digging a trench first. The trench allows you to get deep enough in the snow to begin tunneling into the wall. Shed layers as you warm up to avoid sweating, but always wear a waterproof layer between you and the snow. Even though the digging will cause you to warm up, the most important goal is to stay dry.
The mouth of the cave should be just small enough to fit through to avoid wind exposure. Tunnel at an upward angle so that the entrance is lowest spot of the cave--this will allow ventilation without sacrificing any warmth your body contributes. Also, be aware of how close you are to the surface. The thicker the walls and ceiling of the snow cave, the more insulation it will offer, and the less likely it will be to collapse in sudden temperature changes.
Dig a Cold Sink
The design of your snow cave will determine the lowest temperatures it will reach. By digging a hole deeper than the area you'll be sleeping in, or by building an elevated platform to sleep on, you allow the coldest air to drop below you. The hole, or cold sink, ideally will double as the entrance to the cave so that oxygen can circulate.
Round Out the Inside
Using your gloves, smooth out any rough edges, especially in the ceiling of the cave. The humidity of your breath and your general body heat will gradually cause melting on this inside of the cave and droplets will collect on any sharp points. As miserable as sleeping in snow sounds, it's even worse getting damp under a constant water drip.
Create a Ventilation Hole
One of the most important parts of building a snow cave is also the easiest to forget. Puncture a hole the size of a baseball in the ceiling all the way to the surface. Snow is incredibly good at insulating warmth, but it holds gasses just as well. Carbon dioxide just from breathing can reach dangerous levels without ventilation. Furthermore, if you light a candle or a portable stove, the carbon monoxide can build up quickly. In August 1999, four snowboarders suffocated in a snow cave in Koscuiszko National Park, Australia, because the mouth of the cave doubled as the ventilation hole and falling snow blocked it throughout the night. If you need to sleep in a snow cave, check periodically that the hole in the ceiling remains open. This is especially important if new snow continues to fall.
Mark the Cave on the Surface
The disadvantage of sleeping in a snow cave is that you are nearly impossible to spot by rescuers above. Trample down as much snow as you can in the surrounding area before hunkering down for the night, and leave skis and poles tied to bright pieces of clothing propped in the snow to flag down anyone searching for you.