Select "Impressionist Brush" from Photoshop's toolbox in "Edit Workspace" or right click the Brush tool and select "Impressionist Brush."
Select your desired brush tip -- round or square -- in the "Brushes" menu.
Set the "Size" option by dragging the slider to make the brush tip larger or smaller.
Set the "Opacity" by dragging the slider to make the painted section more or less opaque.
Click and hold on the target portion of your image and move the mouse to apply the Impressionist Brush. Reapply the brush to a single area to amplify the effect.