I use one layer, which is named Tones, to create areas of gray tones inside a selection. The gray tone in the selection on the "Tones" layer is then copied (Command C), a new document is made (Command N), and the selection is pasted (Command V) into the new document. This file is converted to line art by choosing Image/Mode/Bitmap. In the Bitmap dialog box that appears, leave the resolution as it is (the resolution will be the same as the original file, in this case 600ppi) and choose the Halftone Screen option and click OK. In the Halftone Screen dialog box that appears choose a Frequency (lines per inch), an Angle (-180% to 180%), and a pattern (I use Line), click OK.

Now change this Bitmap line art back to Grayscale mode (Image/Mode/Grayscale), then we will delete all of the white pixels. To remove the white pixels double click on the Background and give it a new name (I use the default name), this changes it to a layer, which can have transparency. Now double click the Magic Wand tool and set its options to "O" Tolerance, make sure anti-alias is unchecked, and if your using Photoshop 5.5 uncheck Contiguous. Now click on a white area to select all of the white pixels (if you use a version before 5.5, you may need to choose Select/Similar after clicking with the Magic Wand to get all of the white pixels). Hit the Delete key to remove the pixels and then Command D to remove the selection. Now drag this image back into the original setup and move it into the correct position.

Keep repeating this process to complete the drawing