Now you want to do to the first layer what you did to the second layer.

Enable the visibility of the first layer by clicking on the little box (Item 1 in red above) to the left of that layer's 'Layer Thumbnail'.

Next, select that layer by clicking on its Layer Thumbnail (Item 2 in red above).

Then, click on the tab of the Channels palette to bring that palette forward (Item 3 in red above).

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