I have always gone to the vet's clinic, but if at home, I would try and keep things as calm and normal as possible. Give favorite things if he is still able to eat, if you have to puree steak or fish and let him lick it, assuming he still has interest in food.
If not, just lots of cuddles, maybe wrap a think towel or blanket if he might empty bladder or bowels, use that to cuddle him, and if you can, be there with him. If you cannot, then you vet will be sure it all goes smoothly anyway.
Each dog I have taken was calm and seemed to know what was going on, even those who usually fought a blood-draw just held out an arm and let the doc find the vein, gave a sigh and were gone.
Fran, just make a spot, maybe in the sun, or one of Barney's favorite places, and protect it with a towel or blanket, the vet will know what to do and will guide you.
I'm so sorry you and Barney are going down this road, I so wish it had just been an abscessed tooth!