I'm not sure how much you need instructions, I don't know what they're called, so I can't look for a tutorial. But, they are pretty simple in design- just a double slider zipper* (this is probably the trickiest part- finding a nice one- LOL) sandwiched between two layers of nice wool.
*You'd need the type that opens as the sliders move away from each other, not the type where they open when they move towards each other. I'm sure there's a name.