How do you train a slug?

The simple answer is - you can't ! They lack both the mental capacity to learn, and the memory retention to remember anything you're trying to teach them.

Correction: Gastropods are certainly capable of learning, and there is scientific evidence for it. For example, if you consistently touch the head of a snail, it will eventually stop retracting. In other experiments, a snail could learn to associate a certain taste with a noxious stimuli, so that the flavour itself would cause the snail to retract.