I just did a few signatures on white paper, then scanned the paper and neatened it in Photoshop and inverted the result so that it was white on black then i saved it as a layered photoshop file with the background turned off and the signature as a separate layer in 'lighten' layer blend mode at 20% opacity.
i just drag this layer over to my astro image and job done. it may need the opacity lower or higher depending on the image.
there's probably much easier ways to do it but i am self taught regarding Photoshop
