First you need to have done all the precreasing in step 2. Those are cone bases in all four corners.
Then Step 3:
It is like an inside reverse fold. Focus on the point where the 4 creases meet. The large diagonal is a mountain, the other 3 creases are valleys. The point where the creases meet should be a "valley vertex", meaning that it points down and not up of it. Then just follow the creases and close the model