The best way to transform actual human handwriting into something that the AxiDraw can plot is to capture that handwriting in real time. In real-time capture, you use your computer to record each stroke as it is written, and then save the resulting set of paths as a vector artwork file. This method can also be used when replicating handwriting. For example when replicating historical signatures, it is necessary to trace along the strokes of existing writing and to capture that again in real time.
This page discusses some approaches to real-time capture, as well as some alternatives. Naturally, these same approaches can be used for capturing sketching and other non-handwriting types of artwork as well.
Capturing a signature with jSignature
For very small amounts of text — for example a single signature — a convenient software tool is jSignature. jSignature can create and save an SVG file which can be opened in Inkscape. jSignature can be used from your computer, smartphone, or tablet.
To use it from your computer, sign your signature, and then select "SVG" from the "Extract signature data as" pop-up menu. It should then display your signature as an image below that pop-up menu. Right-click on the image, and select from the menu there "Save image as..." (the exact wording of this option will vary depending on your browser), and save the result with a name like "signature.svg". You can then open up this SVG file from within Inkscape and print to the AxiDraw. For other platforms (smartphone and tablet), you'll need to send the signature to your desktop computer, by a means such as e-mail, before proceeding.
Capturing with a simple graphics tablet
One solution for capturing writing is to use a dedicated graphics tablet such as a Wacom Bamboo or Wacom Intuos Draw. These are relatively inexpensive (< $100) input devices with a stylus. You can use the stylus just like a pen, within almost any application that supports drawing and creating vector graphics. (Be sure to use a vector graphics application such as Inkscape or Adobe Illustrator, and not a bitmap graphics application such as Photoshop.)
While you can certainly write freehand on a tablet like this while watching your screen alone, another approach is to trace existing writing or artwork on a piece of paper. You can attach a piece of paper to the top of your graphics tablet and use the stylus on top of that, to trace handwriting or other artwork.
If you tracing just a signature, it is often helpful to start with an extra large version of your signature (perhaps, the full size of your tablet's input area), and shrink the result down when you are done. This helps to minimize any artifacts from your tracing.
Capturing with a hybrid graphics tablet/monitor
There are also higher end graphics tablets that are actually computer monitors that accept stylus input. A good example is the Wacom Cintiq, available in various models. These tend to be considerably more expensive than a stand-alone tablet, and you may wish to consider a general-purpose tablet computer (e.g., iPad) instead. (Hints on using a tablet computer are in the section below.)
A graphics tablet with a display is excellent for freehand writing, but requires a different approach if you wish to trace existing writing or artwork. Use a scan or photograph of the handwriting or artwork that you wish to follow, and to display it in the background of the area where you are tracing.