This is a slightly more difficult fold but it comes up pretty often in origami. You’re sinking the paper inside the model hence the name Sink.
1) I’m starting off with a Square Base but you can do Open Sinks on all kinds of shapes.
2) I’m going to sink that triangle at the top so I start by folding it down and creasing it well. You might want to flip the model over and fold it the opposite way too, it’ll make things slightly easier.
3) This is the view from the top, open the model out a little bit.
4) As you open the model out you’ll see that there’s an outline of a square. Some of these creases were made when I folded the square base.
5) You want to isolate that square and tuck it inside the model by pushing down on the center of that X.
6) As you push the paper inside you’ll notice that the sides start to come in as well.
7) Push everything closed once the paper is all tucked inside.
This is what the model looks like from the front after the Open Sink.
This is what the Open Sink looks like from the top.
And here’s what it looks like from the bottom which is kind of neat.
If you open the paper up you’ll notice these Mountain (black) and Valley (red) folds.