1. Scan your pages, preferably at 600DPI grayscale.
2. Open levels and adjust (mainly the first and the third sliders) so that the white of the page has little or no dust, while the drawings look as strong as you like them to. For ink or watercolor washes adjust the second slider for subtler control, if you want strong crisp lines, push the right slider until the black of the lines is clear and solid.
3. In channels select Gray (command+click).
4. Select->Inverse.
5. In layers create a new layer and fill it in with Paint Bucket or Rectangle/other shape.
6. Copy the layer or parts of it into your final file or convert the current file into CMYK without flattening layers.

300DPI is usually the minimal required resolution for most publications, however it’s a good idea to keep the resolution higher if your computer allows it. Sometimes the client decides to enlarge the image or it turns out that the printer prefers higher resolution