I'd suggest you buy a plopper copy from Predator Baits, or some other online supplier, so you have a sample in hand when you begin you tinkering. It also helps to know what the plop should sound like. The shape and depth of the tail's lip is critical, but it's easy to duplicate, once you've figured it out.
Please feel free to ask me anything you'd like. Everything I know about making baits I've learned here on TU, from others who shared with me, so it's only fair to return the favor.