hi, this is a great idea to keep you warm while you are camping. it is pretty self explanatory, all you have to do is sew two car windscreen heat reflectors together and put your sleeping bag inside. it reflects most of your body heat back onto you so you keep much warmer.
this cost under 10 dollars, as one of the reflectors cost $7 , the other one i already had, and the string i had as well. i hope this was usefull and it keeps you warm.