Step 2 Click on the thumbnail for the mask to make sure it’s selected. When it is, you’ll see an extra box around it. Now type B on your keyboard to get the Brush tool. Unless you elect to see your mask (see F.Y.I.), it will be invisible. But since it’s selected, you can use it to hide and show the layer underneath. Make sure black is the foreground color in the toolbar, and paint on your image to reveal the layer below.
(As long as your mask is selected, only black and white will show up in your color swatches in the toolbar.)