Nitro
The lead up work is putting the collar on for about 3-4 days and doing nothing. This is so the dog doesn't accociate the collar with the zap, take him for walks and play but don't use the collar. Then you have teach him that a zap means a correction. You don't need a snake for this, you can use anything that you want him to stop doing ie digging, going to close to the chooks, whatever. Next get a dead snake and set him up.

The fact that he takes some steps in direction of the snake would suggest that the correction wasn't enough or you need to re-enforce the training you've allready done. If your collar has a high setting, set it to that. You want to extinguish any thoughts of him following up a snakes scent.