Video: Here's some free advice if you ever find yourself in a post-apocalyptic world and need glasses: Squint. Because after watching How to Make Everything attempt to make a pair of prescription eyeglasses, let me tell you, it's going to really, really hard for you to actually make a pair yourself. There are just so many materials, so many steps and so many places where things can go wrong that getting a lens clear enough and shaped to what your eyeballs need seems impossible.
First, to make glass you'd need silica, potash and lime. Many of those materials you can just grab from sand on a beach but if you forget to filter out something or miss a step, you end up with glass that isn't clear at all and you have to start all over again. But let's say you got the glass down OK. You still have to shape the lenses into your correct prescription, which means measuring the focal length, which means you need to shape the lens, polish it, use a laser pointer, a ruler, a lot of guess work, shape it again, polish it again, hope that you didn't grind too much and... yeah, just squint.
It's still a fun exercise though, so watch How to Make Everything try to pull it off in the video below: