I am so sorry to read about your situation, i sympasize with your situation because I can imagine your situation. There are alot of ways you can create a decent ceremony for your cat. You can print out photos, burry the cat, or cremate it, have a mourning period with candles, sing songs that will help the soul to leave it's material bonds and move forwards.
This is a ritual, that I elaborated for such ocassions, you can use it as a guide.
Before the euthinizing. (bless, and present your cat)
Chant for one who assists anothers death
Goddess of death,
I satnd here as your daughter/son
knowing that life must be winnowed to thrive
This is a wholy act to perform
to open a gateway,
in order to come to you
This is an act of healing,
a release from suffering
an end to pain
Here is one whose arms are open to embrace you
-This chant was taken from "The Pagan Book of Living and Dying" by Starhawk (i have changed some words to fit the description of the above case.
(you can change it as you please, but present the cat to your deities, understanding the cycles of the cauldron)
Create a sacred space for burial or scattering of ashes. If not, just a space to remember the pressence of he who had passed.
1. I take into consideration the fact that this will be an assisted death.
-In the sacred space you can put pictures, favorite toys, favored foods, or scents. Appropriate for the ocassion to call upon the Goddess Baast.
-Create a loving energy as you remember the good moments passed, remind your deceased what their purpose in your life was (how their pressence helped in your growth), be thankful and let it be known.
-Spend those last moments thinking about the cycles of the great mother, how all shall come to be and pass, and come again, and soothe your cat into the great mother, realeasing any bonds that might draw him to you.
-Meditate on life and death, and send your love. This ritual is for your cat and so, everything must be done in his memory. You can invite people to join or just spend this time sensing his pressence and saying all that must be said. And let go with love.
You can leave candles burning with her photo for the time you see fit and wise. As a representation of how she lives on, but still is an undying flame in your life.