Getting snakes started fishing çan be tricky- catching guppies is a skill, but most snakes catch on quickly.
Use a small, very shallow dish to offer the fish, with just enough water to cover the fish.

Crowd the dish- use a gatorade cap if only a few small fish are offered. Your snakes will first go for ones that jump out and flip around, then movement in the dish will attract their attention and they will have a go. As they learn to fish they will become more adept at catching fish in larger containers.

My most skilled fisher uses its own body to restrict the movement of the fish so they can be caught or forced out of the water to be caught, just like using a seine net to trap minnows.

All my garters seem to enjoy the sport once they catch the idea, but only Christmas, a w.c. Eastern, has the skill to chase down the last few elusive fish in a full sized water dish. Butlers are enthusiastic, but not very effective. My onyx checkered had grown so large that guppies are a scant mouthful, but still relished.