I'm sorry you are going through this, and your poor baby too. I have had to euthanize a dog due to spinal problems and loss of bowel control. It makes it harder because the front half might be perfectly normal, but the back end deteriorates. I've got a 13 year old now that had a spinal injury when she was younger, and I will be in the same place as you in another year or so.
When they are euthanized it is basically an overdose of anesthesia- so they go to sleep and just go deeper. Some vets will come to your house, which is nice if the dog is stressed by car travel. If not, some can give you a sedative you can feed them in a special treat so they are heavily sedated when you take them in.
It will never be easy, and you will always second guess yourself. It's better to do it too soon, rather than too late. I once had to have an old horse euthanized; a wise old vet told me to do it when the sun was shining and the old horse was basking in the sun- not to wait till it was cold and the horse was having a hard time with the cold. I did that, and never regretted it. Old Frisky never felt the coming winter in his arthritic old bones, only the warm sunshine and green grass.
Good luck, I know what you are going through.
vickie