Because this bag is such a cheapo piece of junk, the materials used actually ended up costing more than the bag. However, if you are an avid camper and maybe have several really nice bags, then this zip-in liner system can be interchangeable, and possibly extend the life of your higher-end sleeping bag. Its all about better living through doing-it-yourself.
The Liner is designed to zip directly into the edges of the sleeping bag, allowing you to use the installed sleeping bag zipper to vent the bottom if your feet need some air. The liner will also still allow you to zip together 2 identical sleeping bags to make one big one, if needed. The liner can also be removed entirely, and zipped up on its side just like an actual sleeping bag.The other advantage to having a sleeping bag liner is that it can be removed and washed, which keeps the bag itself cleaner.
This instructable is made for a standard adult rectangle bag, but it will probably work with a mummy bag as well, with some modifications.
The sleeping bag used here is Purple on the outside, and yellow on its Inside as a reference.
First, Gather Materials:
1 regular adult rectangle sleeping bag
2 open-end separating zippers, 100-inch length each, preferably with dual pullers.
2.5 to 3 yards of polyester fleece, available from a fabric store