Bert hand-draws a small segment of the fern. Details are added with a thin brush and small eraser. When done, select the drawing, choose Edit > Define Brush Preset. After saving the brush, you no longer need that drawing. The new brush will appear in your brush palette. For a more realistic effect, make another two instances of the fern brush and subtly change the edge details with a small eraser. Save this set as a new brush.