I'm no expert on softies, but I'd tend to put it in a saucer, use a single-edged razor and a quick, sure stroke at a likely spot, like a branch [if I remember correctly what colt looks like]. A rubber band gently applied will encourage a softie to bond to a rock: I've also gotten xenia to do it just by setting a likely rock on their end on the sand.
I'd also run carbon until it heals and bonds---an unhappy leather usually makes everybody unhappy, chemically.