I do not do horns with engrailing any more but when I did, I would cut a groove around the horn after it was shaped. As in tapered, using a Hack saw to begin with and then a Coping saw, sawing down to the depth of the blade, do not remove any material above the cut yet.
Roughly mark out your chips, then using a "V" or "U" chisel (Sharp) depending on what you want, at about a 45 deg angle drive the chisel in so the other side of the groove stops it, one hit one chip. You may have to do some adjustment to get it even, just try and make you layout as close as you can. Once you have come back around the groove start removing the material toward the tip, use a Coarse Rasp with one side ground safe, a 1/2" works great. From there on just shape the neck and tip.
This is a slow process and must be to get it right (Ask me how I know) You may want to heat the area of the engrailing in boiling Water to soften it before using the chisel. Two or three chips at a time is better than continuous chipping.
Hope this helps.
Tim C.
PS: The advice given on the Sibley book is well worth your time.TC