You should declare a method `static` when you don't need an instance to execute the method. Typically, static methods are constructors and utility functions. If the `GameGui` object is in your code and not a third-party library, and `WorldToScreen` doesn't use any of `GameGui`'s other non-static methods or fields, then you should make it static. Otherwise, you should figure out how to get an instance of `GameGui`, perhaps from `GameGui.INSTANCE` or `scene.GameGui`.

Here is an example of a static and non-static method. Notice that the static method `radial` can be called on `Point` directly, but the non-static method `normalize` must be called on an instance.

```csharp
public struct Point {
    public double x;
    public double y;

    public Point(double x, double y) {
        this.x = x;
        this.y = y;
    }

    // *Static* method to create a Point from radial coordinates
    public static Point radial(double magnitude, double theta) {
        double x = magnitude * Math.Cos(theta);
        double y = magnitude * Math.Sin(theta);
        return new Point(x, y);
    }

    // *Non-static* method to set the Point's magnitude to 1
    public void normalize() {
        double magnitude = Math.Sqrt(x * x + y * y);
        if (magnitude != 0) {
            x /= magnitude;
            y /= magnitude;
        }
    }
}

public void demo() {
    Point foo = Point.radial(5, Math.PI);
    foo.normalize();
}
```
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