The Basic Idea:

The valley fold is shown by a dashed line.
Sometimes there is an arrow showing the folding direction.
Fold part of the paper towards you, and crease along the dashed line.

Try to make the edges (or other reference points) line up exactly
(except if the diagrams tell you to leave a gap).

The valley fold gets its name because if you unfold,
the crease will look like a river bottom.