A mural starts as a small sketch with a grid on it. Huge work is small work, scaled up carefully.

Large work is not made large, it is made small and transferred carefully. A grid lets you solve the composition at a size you can take in all at once, then scale it up section by section, which is why enormous paintings come out accurate. Improvising at full size means losing the whole while you fuss over a part. The method is the reason the scale is possible.

Sketch your next big piece at postcard size until it works there. Grid both the sketch and the surface, then paint one square at a time. Step back often to check the whole.