If you have made a selection of the area you want to use as your mask, you can turn that selection into a mask layer very quickly.
- Using a selection tool such as the Lasso Tool, draw your selection in the file.
- With one of the selection tools still active in the Tools Palette, drag that selection from the Design Window into the pencil icon of the image in the Layers Palette.
- Then click on the disclosure triangle beside the image to reveal the newly added mask and make only the mask layer active.
- Remove the selection (⌘D) and continue editing the mask using different brushes and transparencies to obtain the result you want.
Delete
Note: If a mask has already been applied to the image you can replace it with a new one by repeating the steps above. Alternatively, the image can be duplicated and the new mask layer can be applied to it so that you can compare the original mask effect with the new one.