Have to second everything Jayne said. I did this a few years ago with my old horse, she was 35 and had her since she was a foal. She was getting quite frail, and the thought of her getting in trouble or downed during a bad winter was too much. She hated being confined in the barn, so not really an option. We put her down about this same time of year. It was a really nice day, and we all cried a lot.
Husband dug the hole after and put her in it. We let all the other horses sniff her once she was gone. Seems to let them understand in a horsey way, that the other is gone. Seen too many go stupid when the old one just "disappears." Husband is a farrier, has seen the other old horse colic, have to also be put down. Bad enough to lose one, but then the other as well in 24-48 hours from self-induced panic is awful! If they can sniff the downed horse, take their time about it, there never seems to be a problem, no worried horse left behind.
Making it an EASY ending for her is the kindest thing you can do. Our vet did the two-shot thing, it was all very dignified, except for me blubbering.
I called ahead, specified that I wanted the two-shot method because there would be NO MAYBE with a bad reaction. Have heard of horses reacting very badly to the meds of a one-shot euth. Then they needed to tackle the horse to give it another shot while it is fighting and flailing. You never know which horse will have the bad reaction until they get the shot. No chance of that for my girl using the two-step shots.
Hope Saturday is nice and things all go smoothly. Sorry for your loss, she sounds like a good one.