Have a trusted friend come with you. You'll need all the support you can get. How is it going to be handled? Are you going to be holding her or is she going to a vet hospital? If you're going to be holding her, find someone you can count on as backup if you find out that you just can't do it at the last minute.

There's been some great horse memorial threads on this board if you're looking for ideas on that.

Other than that, it's a very quiet procedure. The animal is sedated so it goes to sleep, much like for surgery. Then it gets a lethal injection that stops the heart and lungs. Any movement is due to electrical impulses still left in the nerves and is NOT a sign that the animal is awake/alive. Done properly (and let's face it, it's very hard to do it wrong) the horse lays down and does not get up again.

My condolences. It's never an easy decision or an easy thing to do. Don't be afraid to cry, and it really helps to have a friend (especially one who knows your horse) at your side. Remember the good times, cry, grieve, and take comfort in the fact that everyone losing an animal feels the same.

An old family friend had to put his dog down a month ago, and he simply described it as "it hurts."