On the wall, snap a chalk line even with the windowsill's bottom edge.
Stand the swing stick upright on its arrow-marked end between the window and corner board. The end should be even with the bottom edge of the starter course.
If any mark on the stick aligns with the chalk line, simply transfer all the marks from the stick to the wall. If none align, tilt the stick toward the marks until one lines up with the chalk line. Transfer the stick's marks to the wall.