How to work with Adding Layer Mask
To work with Adding Layer Mask
On the Layer palette, click a layer button to make the layer current (active).
Click the Create Mask button at the upper left of the palette. You have three options:
Left-click to create a mask in the "Hide All" mode.
while left-clicking to create a mask in the "Show All" mode.
Right-click to open a pop-up menu containing the same commands as the Mask and select New submenu.