I have not did this monogram. But, I have pulled up a font reduced or enlarged on copy machine to get it to the size needed....then trace it on a gumpaste placque and then ice over it.
Try a ball tool ---very gentle--- over fresh GP but you need to do it on a foam mat to get a good emboss then let dry.
For me, I would do what Nancyg said with choosing the font and size. And then I would tape the font on the bottom side of glass. Roll the gumpaste out really thin and then put that on top of the glass. Take a light and put it under the glass (you should able to see the outline thru the gumpaste) and then I would trace the outline with RI and then fill it in.