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.