G'Bye ol buddy!!!

It's a difficult thing to do. I don't envy your situation.
I did it with my 17 year old Black Lab/Shepard mix. She had her second stroke in 6 months, and it was apparent that she wasn't going to pull through the second time.
If it's any comfort, the shot the vet gave her acted almost instantaneously. By he time he was through giving her the shot, she was gone. It was very peaceful.

I had her cremated and put her asses to rest in her favorite spot under an azalea bush in the front yard.

My sympathies FF.