Geometric exercises in paper folding (1917)

IIUC this is easy.

Am I crazy or is this the typical 'stuff it into an envelope' fold?

Iterate?

That is, make 1st fold so the edge looks like it is covering 50% of the remaining part of the rectangle. Then try to fold the remaining part in half and see if you were correct.

Not correct? Adjust one way or the other the original fold until you get thirds.

(EDIT: Seems it can be done with some precision if you start with a square — easy enough to get a square from a rectangle. I did the trig though and the technique I saw was within a half-percent of thirds but not exact.)

You could just create an angle in the middle of the edge of the rectangle, so that each line of the angle goes to a corner on the opposite side.

Then use the trisection folds to divide that opposite side in three equally sized parts, and fold it up.

It is also possible to trisect an angle using paper folding too: https://plus.maths.org/content/trisecting-angle-origami

Paper folding is incredibly powerful.