Choose Windows > Editors > Canvas Layer Editor and paint the regions of the mask layer you want to act as a mask.
You can create a mask by painting directly onto a mask layer. Areas of the layer that contain paint will masked, and areas of the layer that do not contain paint will be unmasked. Areas of the layer that contain partially transparent paint will be partially masked.
A new mask layer is created in the lower part of the editor, and automatically selected. The mask is also automatically associated to the image layer you selected in step 1.
See Paint and erase.