I can fold the vertical crease in the center, but after that I have no clue how to fold it back to how it was, along the folds it already has. Is there some method I can use to figure out how to fold it back (preferably one I can use for any paper like this in the future as well)?
Take a good look at the folds: each of them have a direction (up or down). That tells you a lot. You have to follow the existing folds (fold in the same direction).
For instance, the vertical fold down the middle on your photo points down along its entire length. The horizontal folds all alternate. That tells me you have to do all of the vertical folds first, and the horizontal folds last.
There are only a few common ways to fold a leaflet.
- Vertical-first zigzag folding, as you have here.
- Doubling up: fold in half vertically, then fold the stack in half vertically again. This risks making very thick folds, so it's less common.
- Alternating: fold in half vertically, then fold the stack in half horizontally. Even less common, because folding machines don't work like that.
Hobbes (& the comments below) are definitely going in the right direction here, but I think I can figure this particular one from the picture.
First, the verticals are a map-fold (concertina) so you don't fold it in half first you start with the left-most vertical folded towards you, then that pair away, then towards… until you end up with one vertical piece, 6 sheets thick.
Then you fold that exactly in half; that leaves you with rather an odd three-way fold, both folds the same way (vaguely like you rolled it then flattened it).
Trying to explain it more pictorially…
First vertical set is like WW
second fold is a U
Last two folds are both in towards each other (no letter for that;) a bit like an envelope.