That gets me a black and white image on another layer. How to I add that back to the original layer as a mask?
Ctrl+A to select all on the B&W layer.
Ctrl+C to copy that layer.
Go to the channels panel.
Click the new channel icon at the bottom to create a new channel.
Click on that channel to make it active.
Ctrl-V to copy the B&W image onto the new channel.
To make that into a mask:
Ctrl-click the new channel to create a selection (marching ants)
Go back to the layers panel.
Click on the layer where you want the mask
Click the new mask icon at the bottom of the layers pane.
SECOND way (if you don't want to save the B&W image as a channel):
Ctrl+A on the B&W layer.
Ctrl+C to copy it.
Click on the layer where you want the mask.
Click the new layer icon at the bottom to create a mask.
Alt-click on the mask.
Ctrl+V to paste the B&W image onto the mask.