I absolutely love drawing these two. That's a good thing as I'll have to draw them 1,000,000,000 more times over the course of this comic.
I scan the pencil drawing and use photoshop to raise the contrast until the darkest areas appear black. You can do the same with most simple image editors, including instagram's built-in editor. I haven't done any sort of cleanup, I like the rough-around-the-edges look. I'll clean it up a bit before finalizing it for the comic but if it were just a standalone drawing I would leave it as-is. Imperfection can be beautiful :]