You will need:
I use Adobe Photoshop CS. I haven't bothered upgrading.
1. Open the scanned image and your digital drawing in Photoshop.

2. Using the Rectangular Marquee tool, draw a select box over a section of the scanned image from which you want to create a pattern stamp.

3. With the select box "on," select Define Pattern from the Edit menu. The Pattern Name dialog box appears.
4. Specify a name for the pattern (optional) and click OK to close the Pattern Name dialog box.
5. Now pop over to your drawing with the layers, and select the layer where you will apply your new pattern stamp. Make sure that Lock Transparent Pixels (it's a teeny icon in the Layers window that looks like a grey and white checkerboard) is ON. Then select the Pattern Stamp Tool.

6. Select the desired pattern stamp. You might want to increase the brush size, too.

7. Draw all over the selected layer with your pattern stamp.

That's it!
Keet Leibowitz is an artist/illustrator who would like to thank her daughter Maya, age 2, for a growing supply of crazy and beautiful watercolor paintings. Come see our artwork on flickr.