We did this last year with Mojo, our beloved Catahoula. Mojo had cancer near the end of his "poop canal". We waited and let him have his good days and some bad days. He still loved to walk slowly and enjoyed his food. But in the end the bad days overcame the good days.
We decided to have our vet come to the house, on Valentines Day no less. We had Mojo on an old moving blanket. He went peacefully with both of us near. The heavy blanket was a good idea as we had to carry him (around 90 pounds) to the vet's car. Awfully emotional but we were glad we did it this way. I think its better for the dog and its easier not being in the middle of a busy vet office at the end when you are emotionally wrecked.
WE MISS YOU MOJO!