I agree with Turbosaurus. The clove oil method is so gentle, it is truly a merciful way to end your old girl's suffering. She won't be frightened or feel distress when done properly.
Be sure to ask the pharmacist for pure clove oil, they usually know it best as eugenol. I find that it is kept behind the counter at the pharmacy for some reason. If you ask one of the clerks or associates they may take you to the toothache section and give you Oragel, or Anbesol, you don't want either of those.
Turbo's method is easy and doesn't take long at all. If you can find it with alcohol in it then that's a good idea, too.
If you don't have Vodka, however, you can carry this out using tank water, but you must shake the bejeebers out of the mixture so that it appears milky.
I use a turkey baster, reserved for this purpose, and draw up quite a bit of tank water, maybe 3 ounces. I have an eyedropper I got from the drugstore, also reserved for this purpose, and put one drop of clove oil right into the baster with the tank water.
The fish is in a container large enough for it to be held comfortably, but not too big. I shake the baster, fingertip over the opening, and I mean vigorously. Slowly I add this mixture, at the rate of about a drop every couple of seconds, but I shake it up about every 3 or 4 drops. The fish doesn't notice it. Once all of this has been added, I wait.
The fish I have had to euthanise weren't big, so they ultimately were in about 12 to 16 ounces of water, with one drop of clove oil, at this first step.
Usually within 10 minutes the fish has settled down to the bottom of the container and is still breathing, but asleep.
Using the turkey baster I draw up some of the water he's in, I don't know how much really, I just suck up as much as possible, and put about 6 more drops in the baster.
I shake it really well, until milky. Then I add drops very slowly, this should be a lethal dose, so after one drop I watch to make sure there's no movement of the fish, other than breathing. If there's any movement, he may not be fully unconscious, so wait for another 10 minutes after that drop before you continue.
Then, again, shake the mixture like crazy, and add, slowly, about a drop every 5 seconds, shaking in between. Once all the liquid is added back in with the fish this should be enough to have put the fish down. Watch for breathing. I usually leave the fish for a few minutes and come back. If you were to find the fish is still breathing do a few more dops as before.
If you have Vodka I'd go with Turbo's method, but I thought I'd add this as an alternative if you don't have Vodka. You can't use rubbing alcohol, this has to be Vodka.
I'm so sorry for this painful time for you. It really does hurt to lose one our little creatures we've taken care of, and loved.
It's scary and stressful, for you the fishkeeper, especially the first time you have to carry out this sad task, but it can be absolutely stress free for the fish. And, really this is a much less stressful method for for you the fishkeeper than any other method.
It hurts you, but she will not feel any pain or discomfort, just be sure to move slowly, don't rush it. Bless you for caring so much for her that you want to do this for her.
EDIT: I'm sorry I posted this late, though you had excellent advise to help you do this. I'm so terribly sorry for your loss.