1.

For an example, we have a model shown on the left. Here it is a kite base folded in half and we want to apply an outside reverse-fold.

2.

A typical diagram of outside reverse-fold consists of a valley-fold with two arrows which is shown on the left at (A).

But I think a valley-fold at the edge replacing two arrows is very simple to indicate an outside reverse-fold.

3.

Now, let's apply the outside reverse-fold.

Make sure that all three crease lines are valley-folded.

4.

After you completed applying the outside reverse-fold, you will get the one shown on the left.

5.

In a typical diagram, only two valley-folded symbols are given (shown in (A)) and you should be able to apply the outside reverse-fold to get to the picture shown in (B).