I've had to do it four times, it's horrible. My last dog was 14.5 and was loaded with cancer. The vet said in 30 years, it was the worst he had ever seen, and if he was a person, he would have been screaming for morphine or to die.
He didn't seem to be in pain and was eating ok, but the vet said he would be shocked if he lasted two weeks. We took him on a "farewell tour" and got him all the ice cream and burgers he loved so much and to see all the people that he knew over the years. Ten days later, he started acting very strange and it was time. Odds are, in about 5-7 years, I'll be doing it two more times, and I'm dreading it. You're doing them a favor, but it's hard as hell to do it.