I've never had to put a horse down, but I've been there for several friends who had to do it. We started the day by brushing the horse and making his last hours as comfortable as possible. The vet came, we led the horse to the prearranged spot (usually soft grass), and either we stood silently or the owner had to leave and I stayed with the horse.

I know I'm going to be a wreck when the time comes for my little mare. I hope I can be with her to the end, but if I can't, I have several friends who will.