This is another very common fold in origami. It’s just like the inside reverse fold except the paper is folded on the outside.
1) I’m starting out with a square folded in half diagonally.
2) I’m going to do an outside reverse fold with this little flap here so I start by folding a Valley Fold.
3) Just like with the inside reverse fold you want to fold it the other direction on the other side too.
4) Once you’ve folded both ways along the crease open the paper up. You can see how the corner kind of wants to pop forward there.
5) Fold it back in on itself along the two creases you made.
6) Here you can see what I mean by folding it back on itself.
7) Push everything nice and flat and you have an Outside Reverse Fold.
If you open the paper up you’ll see the following Mountain (black) and Valley (red) Folds.