i had to preform a ceremony for my friends cat this year, but i didn't keep the ceremony i typed up, so i don't remember the words.
basically, you need a bell, a picture of your cat, a plant, cat treats, and a cat shaped candle [or a candle of similar colour to your cat]
say something welcoming everyone, and why you are here. say a prayer to Bast [it was along the lines of 'Houdini is now in Summerland, by your feet Bastet, protect and watch over her, as we have done'] ring the bell 3 times and light the candle, say how the candle is your cat, and that he/she shall return when the candle is lit, until it is gone or you have finished grieving. [if you don't want to have your cat attached to the candle, as my friend didn't, you can change the words. i think it's more of a comfort thing, but if you differently, change it around] after that, we all told a story about the cat. we then opened the circle and threw cat treats in all four directions as offerings.
the plant normally is buried on top of the animals grave during the ceremony, but if the cat's cremated, just take it home and care for it, it's a symbol of your cat living on, just as the candle represents your cats spirit. sorry i couldn't remember the whole thing, just do a google search, you'll probably find it or something similar.