Using a sketch as a base - how to do line art over top

I use work paths.  They are east to make totally accurate with smooth curves, and can be adjusted as you go.  You can then build the finished art work a layer at a time using those work paths.  This is the way that Bert Monroy does his digital portraits as in this detail shot of Bert's Times Square Gigapicture

You can download the notes from Bert's digital art workshop he did at Max last year here

This portrait of my wife is 100% digital art, and was produced using Bert's techniques.

There are just under 100 layers, but many of them are Smart Objects with multiple layers, so the total count would be a fair bit higher. The broach and earrings were produced separately using Photoshop's 3D tools.  A good 90% was done with brushes from this brush set by Castrochew, but I made other brushes for stuff like eyebrows using Fade to taper out the lines.   If you are using Windows, then Lazy Nezumi Pro is a huge aid.  A tablet is an obvious must have.

If your (photographed) drawing is a black line on a light background, you can open the photo creating a background layer, and then duplicate that background layer.  Working on the duplicated layer, go to Select/Color Range and, go for Shadows in the drop down list, and adjust Fuzziness and Range till you get the black line drawing selected.  Then invert the selection (Select/Inverse), and remove the background around the line drawing.  You will then have a layer with nothing but your line drawing.  If you then create a new empty layer below the line drawing layer and fill it with white, You will see your original line drawing against a white background.  Add a new empty layer on top of the stack and work on this with your colour work etc.  I'm assuming you want to develop the line drawing in your photo.

David