Now, you want to create what in essence is a brightness selection. This is done with a simple
Command-click (Mac) or Ctrl-click (Windows) on the icon of the RGB composite channel
(Item 1 in red above).

Once your computer has made the selection (the selection has been made when the lines of 'marching ants'
appear), click on the Layers palette tab (Item 2 in red above) to bring that palette forward again.

Click 'next' in the top right corner of this page to move to the next step.