I unfortunately have had to have 4 horses put down, all for different reasons. It doesn't get any easier that's for sure.
Each time we had the vet use the "Green Dream" injection. He would first sedate them if they were standing then administer the injection. It was like they just fell asleep, I could never shoot one of my babies as it would be too traumatic for me to handle. I know that sound selfish, but that's just how I am.
I still cry for months afterwards no matter what.