STEP 2: CREATE A NEW BLANK LAYER
Press F7 to open the layer window. Make sure that the left glass is selected, press ctrl+shift+N/cmd+shift+N to create a new blank layer. You can also add new layer with the layer window just by clicking on add new layer at the bottom of layer window.
After creating the new layer, double click on its name and change it to left. You will not see any difference in your photo because the layer just created is totally blank and totally transparent.
