There are two essential masking techniques.
Here is the "Reveal All" approach:
- Make sure the proper layer is selected in the Layers palette. (Actually, you can mask any layer other than "Background".)
- In the Effects palette, double-click the "Add Layer Mask" thumbnail. The mask thumbnail appears in the Layers palette. Its white color means that the entire layer is revealed (not hidden).
- Now to hide some area of the layer, you paint over it (or fill it) with black color.
Alternatively, you may use the "Hide All" approach:
- In the Effects palette, double-click the "Hide All" thumbnail. The thumbnail in the Layers palette becomes black and the mask hides the whole layer.
- Now paint white color to reveal (unmask) a portion of the layer (screenshot).
Sometimes, though, it is more convenient to create a mask from an active selection. That is, you make a selection, for example with the Polygonal Lasso tool, and then use either the "Reveal Selection" or the "Hide Selection" action.
To apply a layer mask (i.e., to merge it into layer content) drag the mask thumbnail onto the "trash" 
button and then, in the dialog box, click "Apply". To discard it without applying, click "Delete".
