This seems simple but if you don't know exactly what you're doing and learning as you go (like me) it's easy to mess up. I have been working on my first digital children's book and fortunately the company has been working with new illustrators and has been very understanding.
What I had been doing
Scanning and then desaturating and adjusting levels, making the blend Multiply, but when I needed to edit my line drawing, I both drew with my tablet and scanned additional lines. Well, what I learned was that this makes a huge mess! Each time you adjust the levels it affects the pixels and the more you do it, the more pixelated your lines can get. So that's my problem?! Though the "multiply" layer method is the standard, I like this one better because you just get your outlines rather than a white background with it and the author talks about being able to work with transparent pixels which is a future lesson for me but sounds handy.
So now, rather than beating myself up for making so many mistakes, I'm realizing how much I have learned and feel confident that my second book which I am just starting is going to go even more smoothly!
What I have learned
I am so relieved to have found this information that I wanted to give them a shout and share with people like me. This link is to a blog called Chopping Block. It explains everything and then includes a free downloadable Photoshop Action. I first went through the instructions manually so I understood how to do it and then I tried the Action that I downloaded. It is slick! And I anticipate it will make my future line work much better.
I am in their debt.