- Use adjustment layers to begin cleaning up your line art.
- Remove any non-repro blue underdrawings with a Hue/Saturation adjustment layer: choose the Cyans colour range in the Properties panel and set the saturation to minimum and lightness to maximum.
- Use Curves and/or Levels adjustment layers to remove any remaining underdrawings, and to improve the line quality and whites.
- For high-DPI inked line art, use a Threshold adjustment layer to convert the line art to black-and-white bitmap. This makes flatting easier.
- At the bottom of the Channels palette, click on Load channel as selection to create a selection of the white areas of the image.
- Invert the selection.
- On a new layer, use the Paint Bucket tool to fill the selection with black.
- Deselect the selection, and hide the scanned artwork and adjustment layers. (I prefer to hide these layers instead of deleting them so I can refer back to the adjustment values I used.)