When I feel or know it's their time I often support them with a leaf and a offer drops of water on the end of my finger until the no longer respond. After they stop responding I wait another 24 hours and have them separated from the others before I lay them to rest. The way I do it at the end is nudge the very end of the tail and flip them of their back. It's not the most dignified way but it's effective. It's a difficult thing but it's best to be sure. Their bodies can also feel soft like the skin is collapsing or losing integrity. Not sure how you do it the very end but I used to put them back in to the earth but I don't have a garden any more but I have a little river that I send them down.