When a player is on the ball, imagine a triangle with the ball and the goalposts. In most cases, you want to be in the center line of that triangle, just far forward enough to be able to reach both sides of the triangle with a collapse dive (in which you take a large step with your leading foot and shoot your lower hand through your target). If you’ve a rope and a partner, you can physically create this triangle and practice positioning by trying the rope to both posts, having the partner pull it taut and moving about. At any given point, you should be able to reach the rope on both sides, without being so far forward to allow a ball over you.