01. Select the 'Brush' tool and go to Window --> Brushes (or F5). Go to Brush Tip Shape. Set the 'Diameter' to 25px, the 'Spacing' at 5% and the 'Hardness' to 100%.
02. Go to 'Shape Dynamics'. Set the 'Size Jitter' to 100%, the 'Roundness Jitter' to 75%, the 'Minimum Roundness' to 25%, and check the 'Flip X Jitter' and 'Flip Y Jitter' boxes.
03. Go to 'Scattering'. Set the 'Scatter' to Both Axes and 38%. Set the 'Count' to 5, and the 'Count Jitter' to 0%.
04. Take the image you will be applying burnt edges to. In this case, a brown rectangle with 'Stroke' applied.
05. While holding shift (to paint in a straight line), paint along the edge you wish to apply the effect to.