The Basic Idea:

The mountain fold is shown by a dash-dot line.
Sometimes there is an arrow (with a half-triangle tip)
showing the folding direction.
Fold part of the paper away from you,
and crease along the dash-dot line.

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

The mountain fold gets its name because if you unfold,
the crease will look like a mountain ridge.