Download a free vector graphics program called Inkscape (very similar to Adobe Illustrator). Go to File->Document Properties and set "default units" to inches, so the rulers along the edges and cursor position shown in the bottom right corner on the status bar will be in inches (of course you can also use mm if you prefer). Drag and drop the photo file from windows explorer, and rotate and scale until it's the right size (make sure you scale uniformly by holding ctrl while dragging the handles). You can also use Object->Transform to scale numerically.

Then trace it using the tool "draw bezier curves" (keyboard shortcut shift-F6) and "edit paths" (shortcut F2), and Object->Fill and Stroke to edit the line width and color (set fill to none if it isn't already). When editing nodes, clicking on them while holding various combos of ctrl, alt and shift does different things. Just fiddle around with it until you get the hang of it.

Most guitar shapes can be produced with 8 nodes (two each for widest point of the shoulders, narrowest point of the waist, and widest point of the lower bout, and one each for top center and bottom center) and the node handles all going straight up/down/left/right. Hold ctrl while dragging a node handle to snap the angle.

You might be able to fit just the body on standard printer paper, but if you want to do the whole thing you can save to a thumb drive as PDF and take it to Kinko's or similar to get it printed. Since all the positions are in real-world units, it should come out just right. The File->Document Properties window also lets you set the page size shown in the editor, which is helpful to see if everything will fit on whatever size paper the print shop has.