when you buy chlorhexidine it will either be at 2% concentration, or 4% concentration. (4% concentration is harder to find) it must be further diluted to make it safe but still effective. as John1982 said 1 oz of solution per gallon of water is usually recommended.
A shot glass (ounce) or so per gallon. My Vet says they use it at different concentrations for different applications like sterilizing surgical equipment or cleaning out a wound. But like snake lab I just kinda wing it . I do make a stronger mix for my water bowls but make sure to rinse thoroughly.