Step 6 - Making a line layer

In this step, we'll subtract everything but the black lines from the Line layer.

Select the Line layer in the Layer window. In the color swapper, swap B&W again so that this time black is on top.

Go to Select>Color range. In the popup, turn the Fuzziness slider all the way up to 200 and click ok. You've just selected all the black in the image. Now, go to Select>Inverse. This command selects everything in the image that you weren't selecting before.

Finally, push the Backspace button. This deletes all the white in the image, so that all you have left is the black lines. To see it in action, make the Color layer invisible by clicking the eye next to the layer. You should see just the black line of the Line layer over a checker pattern (which represents transparency in Photoshop).