Let's construct a tube tent first. Place 1/3rd of the tarp over the site where you want to sleep. Tie a line from the tarp to a tree on either side of where you will be sleeping at about 2/3rd of the tarp width. This will form the ridge of the shelter, called the ridgeline. The last 1/3rd of the tarp is brought down over the bottom section of tarp. Make sure that it extends over the outside edge of the bottom tarp in case it rains. You don't want all the water from the rain falling on the floor of your shelter. Make sure you tie the corners of your shelter down and taught to prevent the shelter collapsing during the night if it is windy. You can close up one end of the tent to make it a little cozier.
If the eyelets of your tarp have torn off, how do you secure the lines to your tent? Take a small rock, approximately 1" in diameter and place it on the inside of the tarp. Crunch the tarp around it. Now tie the line around the rock on the outside of the tarp capturing the rock on the inside of the tarp. This should secure the line to the tarp without needing to cut any holes in the tarp which will likely rip out over time.
You can build up debris on the outside of the tarp to help increase the insulation of the tent when it is cold. You also want to make the volume of your tent as small as possible in really cold weather to decrease the amount of volume your body will be trying to heat. What should you sleep in? Put on as many layers as you can, this will help you stay warmer. If you packed an emergency space blanket you can wrap yourself in that. Another lightweight option you should carry with you is an all-weather emergency bag which is made out of space blanket material. The metalized mylar will reflect your body heat back to you keeping you warmer.
A tube tent is a quick to set up, simple lightweight shelter to carry with you. It consists of a plastic tube that you run a line through. You tie the line to two tees to form a ridge line for the tent, similar to setting up a tarp tube tent. Close up one end and crawl inside. Remember to use debris under the tube to help insulate your body from direct contact with the ground which will rob your body heat.
Another form of emergency shelter has a single fold on the windward side, with half of the tarp on the ground and the other half forming the shelter roof at a 60° angle. The ground sheet will need to be staked to the ground preferably over a pile of debris to act as an insulating layer. The roof will need to be tied taught between two trees. On the inside of the roof you could add a metalized space blanket which will reflect heat from a fire behind you when you are lying down. The space blanket can be held in place using a twisted loop of 550 paracord which is strung across the top edge of the emergency shelter and tied between the two trees. The space blanket is held between the two strands of paracord which are twisted together. The other side of the metalized mylar is attached in a similar fashion near the fold in the tarp. Place the fire in the center of the opening of the emergency shelter but far enough away as to not present a fire danger. Make sure the fire is well contained in a ring of rocks and that there is no combustible material nearby. Debris can be used to create side walls to the shelter as well as adding a layer to the outside of the tarp on the windward side.
If you don't have a tarp, you could create a ridge line with a stick held up by two forked branches or between two fairly close trees. Additional branches could be placed against the ridge line stick at a 60 degree angle to form the roof. Boughs of evergreens could be placed upon the roof supports to act as a roof to protect you from the wind and rain. A debris pile can be placed on the floor of the shelter to insulate you from the ground. This is probably one of the easiest shelters to make if you don't have a tarp.
