As stated, the best way to dehorn them is by using a polled bull.
But... as you said, you purchased them this way. Dehorning an 11 month old - unless you're just tipping/blunting the horns, is going to open up holes into their sinuses. Depending on breed, this may be a whopping big hole that's going to take a long time to cover over, and in the meantime, they're gonna be subject to infection, so be on the lookout for purulent exudate (pus) coming out of the open hole &/or their nostrils.
Disbudding with a hot iron dehorner/disbudder is my preference, and the sooner you do it, the better.
'Pulling' arteries means grabbing that spurting artery with a hemostat or a pair of needle-nose pliers and pulling it so that it either breaks off below the skin &/or causes a vascular spasm, shutting off blood flow from the severed end.
I dehorn so few (less than a half-dozen/yr) anymore that I always do a cornual nerve block - which I rarely did when I was in veterinary practice - so there's little to no acute pain when I'm burning or cutting off horn buds or horns.
Pine tar... that's a throw back to a time when we had little in the way of effective fly repellents. But, Yes, I have smeared pine tar on dehorning and castration wounds... back in the 1970s, before I went to veterinary school. I'm not really sure how effective - if at all - it is as a fly repellent, and I can almost assure you that it has an adverse effect on wound healing... there is great truth in the old adage, "If you wouldn't put it in your own eye, don't put it in/on a wound."