If, like me, you think traditional printmaking is awesome but don't have the manual dexterity to carve out fine channels in wood, a laser engraver provides a handy shortcut. Here's the process.
Step 1: Create your file.
To start with, you want a file that's pretty close to monochromatic. This can be a digital version of an existing copyright-free woodcut, a scan of a pen-and-ink drawing, anything really. In this case I'm using a digital image of a woodcut by Leonardo da Vinci. You're going to want to do three things to it: first, invert the colors if necessary so that it is light where you want to print and dark where you don't, second, flip it left to right (if you care; some things look just fine mirrored) to create the version you want to carve (final prints will be a mirror-image of the block), and last, turn it into a file that will work out well on the laser.
The first two parts of this can be done in any paint program. Save the result, and import it into Inkscape.Select it, and under the "Path" menu choose "Trace Bitmap." Select the "colors" radio button and adjust the "steps" option to 2. You will now have an additional object sitting on top of what you imported. In creating it, Inkscape will have done a good job of adapting the shape into vector form so you can scale it smoothly, as well as converting "really dark" and "really light" into one color each. Ungroup that (under the "Object" menu) and drag the two pieces of it apart. One of them is the dark shape you want to print; the other is going to be a light-colored box, which you can delete.
Delete the original imported version of your image, leaving only the vectorized thing you want. Adjust the fill color on that part to be black. Re-scale the paper to fit the drawing (Inkscape has a button to do this automatically under "File" > "Document Properties"), then add a box around it that has the same proportions as the image, no fill paint, and a stroke width of .001". Save your file.