We do a lot of them...we are a "vellum" shop...I provide client a stapled sandwich consisting of an 11"x17" piece of vellum pack backer cardboard, a grayscale copy of art at final size, and blank vellum on top for the kids to sign. used to provide scrapbooking lightsafe pens, (nice even Black signatures) got sick of them not coming back, or ruined if they did...Client can use what ever they want..I tell them to use a sharpie...Instruct to sign inside design...Scan into p-shop at (at least) 400 lpi...Any signatures that are obviously too light to work properly, I will usually take my "burn" tool, and darken them up...If too small to hold detail, I will cut/paste/scale as needed...make B/W tiff...no need to trace...pop into Corel, scale, and, voila!