I've been looking for a paperless way to do this, and I've found a fairly easy way to do it using the GIMP (http://www.gimp.org) and OpenOffice (http://www.openoffice.org).
- Scan your actual signature and edit it in GIMP, converting the background to transparency using Colors -> Color to Alpha, and then save it as a PNG file. You may need to rescale your signature to have it fit on documents, depending on the resolution of the original scan.
- Download and install the OpenOffice PDF import extension: http://extensions.services.openoffice.org/project/pdfimport
- Open your PDF document in OpenOffice, which should open in the Draw program.
- Select the Insert -> Picture -> From File... menu option and select your signature image.
- Move it into place on the document.
- Print the document and in the Print menu, select "Save as PDF"
- Send it via e-mail!