2. Make art for a custom brush: Using the brush tool with the smallest brush, make a series of vertically spaced dots like the enlarged example to the left. Apply the Blur filter. Using the Rectangular Markee tool, select the image you just created; go to the pop-up menu in the Brushes Palette and select "Define Brush." Your new custom brush now appears in the Brushes Palette. You no longer need the brush artdelete the image.
3. Go to the Channels Palette and create a new, empty channel. Go to Image > Adjust > Invert (Shortcut: Command-I). Choose the custom brush you created in step 2 above and, with black as the foreground color, draw a horizontal stroke from one side of your image to the other. (Hint: hold down the shift key as you draw to constrain the stroke to a horizontal line.)
4. Continue to draw additional lines until you have the amount of grain you desire. You may overlap the strokes to make the grain denser or more complex.