for whatever it's worth, here's my two cents worth: (one guy's advice: free and no doubt well worth the cost)
take the horn surface to the level you want (scraped, sanded to 400, 600 or whatever grit surface you like) ... then put a light spray of adhesive on it and let it dry completely.
now the 'hard' part ... this involves a #2 lead pencil (sharper is better) and the application of patience. the spray will give you enough 'tooth' so that you can easily sketch on the horn. a rubber eraser may also be required ... repeat and correct as necessary ... if you do it this way, you will never have a horn which looks as though you used el- cheap- o 'clip art.' it will, instead, be a design of your own hand: undeniably your own work. to those who would say, well he copied thus-and-such, I would answer NO - he was inspired by thus-and-such, or he wanted the horn to look like thus-and-such.
when you're happy with the result, respray the horn with the magic rattle can and - -viola! your design will stay put, and will not smudge when you handle the horn as you do the scrim.
when you're ready to apply the ink, just go over the horn with some 0000 steel wool or superfine wet/dry sandpaper to remove the spray.
this is not, I will the first to admit, very HC/PC. it does, however, work very well for me.
good luck with your project and, as always, Make Good Smoke!