Most tents do not come with insulation and are not meant to keep campers warm. They are instead designed to protect campers from the wind and rain. Unless a tent is a "four season" tent rated for winter camping, campers will need to add insulation to stay warm and toasty on a cold weather camp out. Insulation is primarily used between the camper and the ground, and can cover just the area underneath the sleeping bag or the entire tent floor.
Place a tarp under the tent to keep moisture from seeping through the floor. This will also help to keep the tent clean. Use a tarp the same size as the tent floor, or fold the tarp underneath the tent--do not allow the tarp to stick out from the tent as any exposed sections will catch rainfall and direct it right under the tent.
Cover the floor inside the tent with an insulating layer of blankets or cardboard. Space blankets make an excellent insulating layer since they are lightweight and fold up very small. Conventional wool or fleece blankets are also very warm, though bulky to pack.
Protect the tent from wind with a wind break. This can be a simple tarp hung from a line strung between two trees and staked to the ground, or it can be a natural wind break formed by trees, a hill or large rock outcrop.
Use a rigid foam pad under the sleeping bag to further insulate the tent floor. Don't use an open cell (soft spongy foam) pad as these will compress under the camper's body weight and will not provide warmth. Air matresses also do not provide insulation to keep campers warm.
Make sure the tent is properly ventilated to keep moisture from your breath from collecting inside the tent. Most tent doors do not completely zip shut at the bottom, which allows for ventilation. If the tent does not have this option, keep a door or window open a few inches to provide an escape for moisture. Bring a small wisk broom to brush snow off gear and clothes. Not only will this keep the tent cleaner, it will also keep melted snow from collecting inside the tent and forming a puddle.