Step 7: Fancy trickery
Now it's time for some cleverness with layers. Click on the little box with two arrows in the bottom left corner of the layers palette. This should make the advanced layer settings (Opacity and Mode) visible.
At the moment, the layer called Background copy should by in Normal mode. Use the drop-down menu to change it to Add mode. Everything in your image should turn white. Don't panic! That's what we wanted.