My condolences.
When it was my Nik's time she had let me know, now it was time for me to honor the 18 years she'd been a loyal, loving member of my family. I had a vet come to the house, a beautiful day in early May. We, (me, my 2 dogs, Niki and the vet) all had a 'picnic' outback, Nik had some tuna juice and a some water. We said our last goodbyes and I held her while she was given the shot, she passed in my arms. Hard as it was, it was the kindest thing I could have done. Niki's ashes occupy a place of honor on a shelf in my bedroom.
I have tears running down my face right now. I just relived what you may experience tomorrow. Stay strong and be with her till the end.
You'll be in my prayers.