Q: I do not understand the symbols in your diagrams. Can you explain their meaning?

We use the standard Origami symbols.

Valley fold means that the fold creates a valley-like bent in the paper.
The Valley fold line is marked with a dashed line.
A full head arrow shows which point moves to where. The arrow tail show the point that should be relocated (top right corner here) to the point that marked with the full arrow head (left lower corner).

Mountain fold means the folded paper creates a mountain like shape.
The Mountain fold line is marked with a dashed line with two dots between each dash.
A hollow arrow head represent this fold. This arrow does not show which point moves where.

This symbol ask you to turn the paper over. The direction of the arrow is important and represent if the paper should be flipped from left to right, or upside down. Here the left side turns to be on the right side.

Two arrows represent the Fold and unfold steps.  A folded line that was unfolded is marked with a thin continues line.