Alright, so open up the scan you want to extract, and make sure you have the following selected:
Note: make sure your color palette is set at the default setting. To do this, press the "D" key on your keyboard.
Next, duplicate the "background" layer (right click on "background" layer and select "duplicate"). You now have 2 layers: Background and Background copy.
On the "background" layer, do Edit > fill > foreground color (which should be black.)
Then, go back up to the "background copy" layer and click on little button at the bottom of the layers palette that looks like a white circle in a gray box. That'll creat a mask layer.
So now, you should have this:
Now to start extracting. Take a look at this:
Please note that the mask layer is the one selected. Do everything on that layer! To select the mask layer, simply click on it.
Next page!