If SHTF, then we should think about old-school methods based on animal extraction. The process is involved, but still approachable.

My daughter did a project in her high school chemistry class on the entire process, describing the steps involved - I don't have that paper handy, but if a high school student can work it out it's probably available elsewhere. She later told me that this project was part of her post-apocalypse-family-survival-plan;)