Of the setup you need:
Set the compound to the taper angle you want, in this case, I have a 10 degree taper set for this muzzle brake, that will give you a 20 degree included angle. As you can see, the cutter is set perpendicular to the compound. You'll most likely need to make several passes, advancing the cutter with the cross slide, but actually making the cuts with the compound. Stop when you have the small end to the size you want. It sounds more complicated than it is, once you've done the first one, the rest are easy
BTW, you're probably better off starting with a wider angle than you think you'll want, that way, if the taper ends up shorter than you want, you can just decrease the angle and re-cut it. I happen to like 10 degrees, it gives me a nice looking taper without being too long. Hope that helps.
PS Something to keep in mind, on my lathe, the cross slide is at zero when it's parallel with the spindle axis, on some lathes, that would be 90 degrees. Obviously, if yours is the latter, you'd want the compound to be at 80 degrees in this case, rather than 10.
Oops, that should say the COMPOUND is at zero, not the cross slide. Sorry if that caused any confusion.