Essentially, the killing must be both humane and using an appropriate method. Simply being humane is not sufficient to avoid the risk of prosecution and I doubt that anything except using drugs only available to a licensed vet is likely to be deemed as appropriate for a domestic pet. My local vet does home visits for euthanasia, for those of us who don't want to stress the cat out with a visit to the vet.
You have my sympathy for the loss of your cat. I've lost three since November, only one of which died in its sleep.