I chose to use The Gimp for this guide, since it is freely available. Someday I'll update this for Photoshop.

First off, I have to scan the image. I always go for at least 300 DPI (Dots Per Inch) and greyscale so I have a resolution high enough to work in. For professional work I may go as high as 400 to 600 DPI. Once an image is acquired from the scanner, I have something like this:

Once I have an initial scan, I need to get the lines black, and the whitespace white. To do this, you could play with the brightness and contrast, but I've found that modifying the levels gives you much more control. Simply right click the image to and click Image > Colors > Levels... and a dialog similar to that pictured should appear.

I usually go with something like 120 and 180 to get good contrast for my inking, but this will make pencil work look terrible. Play with the sliders and figure out what fits you.

At this point I usually convert the image to RGB mode so it will be in a format that can take color. This is easily done by right clicking the image and then selecting Image > Mode > RGB, or you could simply press Alt+R.