How to mask layers in Photoshop

Basic principals
Masks are used to ‘mask’ elements or layers.

The first step is to click on the mask icon. This will create a new mask icon next to the layer icon.
You can select the icon for the layer which allows access to the RGB colors. This means painting, texturing, anything to do with the visual color space. The Mask Icon and the Layer Icon are both selectable. When working on the mask, be sure that it’s icon is selected. Don’t forget to RE select the layers icon when you want to edit the color space.

Masks in Photoshop work off a black and white color space. The black and white color space also include gradients of black and white. For instance a 50% grey is interpreted as 50% opacity.

Selection

Masks will be created best if you have a selection already active. Selections can be made from bumping shapes, using the marque tool, lasso tool, creating a Path, or using the Magic Wand selector.