The second way to do this is as follows.

Follow the same process as before so that you have a pasted layer with a white background above your parchment layer. However don't set the layer mode to multiply.

Before going further, use the layers dialogue to correct the whites as described above. Now we are ready to move on.

With the scanned image selected go to Layer->Transparency->Color to alpha...

Make sure that the box beside "From:" is white. This will turn all instances of white on this layer into a transparent image. Now you see why we needed to have the background as pure white as we could. Press okay and you will see your white background magically disappear. I prefer this to the multiply method because you aren't affecting any other layers with this. Multiply does affect the layers below it so it can create some wierd glitches further down the line. Plus it will be useful to be able to mess with the line layer modes later on which we can't do if we are relying on them being set to multiply at this stage.

This should give you an image much as before:

example2.jpg

Now this is a little light so we'd like to make it a bit darker. With the scanned layer selected press ctrl-shift-D or go to Layer->Duplicate Layer. This should darken the lines nicely. Play with the opacity of the new layer (using the opacity slider in the Layers Dialogue) until you decide it is dark enough. Then right click on the duplicated layer and go to Merge Layer Down. This gives you a lines layer as before. This now looks something like this:

example3.jpg