When counterfeiting a signature for a prop it's best to get a piece of scrap paper and copy it many times learning the ebbs and flow of the signature as it's drawn.
I say drawn and not written as your copying an image ultimately. You can enlarge it and put it against a (graph paper) grid to learn the exact spacing of the lines if you need to understand the writ to such a level for making multiple props where believable differences are needed.
If you only need one, you can print a super light gray (watermark) thin signature copied from the original and write over it with a pen/quill/marker/etc and it should satisfy you.
Real ink, ballpoint ink from older pens have a sheen water based inkjet printers can't produce. The imprint (or lack) of the pen as it's written is obvious when looked at.