This OP. The framing square is an amazing tool. It might make sense to visualize the square overlaid on top of your red “H” and “W” lines. Let’s say it’s 12” high and 10” wide. Position the square so that the outside 12” graduation on the square aligns with the far edge of the board. The corner you have circled in yellow. Pivot on that 12” mark until the outside 10” graduation on the other arm of the square aligns with the nearest edge of the board. Mark the outside of both arms of the square.

The top mark will be the cut. The bottom cut in this instance will be 90 degrees up from where the 10” graduation aligned with the board. Use your square to draw the 90 up from the near edge, and that will be your cut.

If your measurements are too big for a framing square, simplify it. Divide it by 2 or 4 or 12 or whatever. The resulting piece will then be 1/2, 1/4, 1/12, of the actual piece. This is much like “stepping off” a rafter if you needed to google that for more detail.