I have had to do this twice in the last 2 years.
The first time I was well prepared, I knew I only had one more day with him and phoned the vet when I saw that the time had come. They were very kind and sedated him with valerian, he was normally a fighter but he lay there while he had head rubs, then sedation. We were left at some stage to say goodbye before the vet returned and injected him. He went peacefully.
The second time was only a few weeks ago, final tests showed something that could not be treated, however the results came on the day I had decided we needed to take her in so we were told when we arrived which didn't give me time to prepare and take my time to say goodbye before we left. It was a different vet from the time before and it was an earlier appointment as it wasn't expected so didn't have the luxury of extra time that you sometimes get at the end of the day. She was given sedation while standing up and I picked her up and cuddled her on my lap while the final injection was given.
No tubes, just normal injections for both of them. The first time I refused to cry, I wanted the day to be normal for him so he felt no stress, it was good and how I wanted it to be, my only regret was not being able to hold him but he knew I was there. I had a tear run down my cheek as we took him out of the room. I broke down later.
The second time I sobbed as I watched her trying to stand, didn't hold it together at all but she was in my arms at the end which is where she always wanted to be.