Create new layer, select Brush tool (B) and in Window menu, open Brushes tool window. Select our new brush, then go to Brush Tip Shape and increase Spacing to some 500-600%.
Then, enable Shape Dynamics, and increase Size Jitter, Minimum diameter and Angle Jitter.
Then, with black brush, make some chaotic strokes to put some leaves nere and there. It is important that leaves are not overlapped.
Now we have some black leaves, but I want brown ones. To fix that, use Magic Wand to select empty area of layer with leaves...
..then invert selection (Ctrl-Shift-I), create new layer, and fill selection with brown color.