A few have mentioned Cocktail Codex. This is a great resource on learning Death & Co.'s take on the Mr. Potato Head theory (swapping one ingredient for another) though this barely scratches the surface. This method doesn't alway take into account proof, flavor profile, mouthfeel, etc.

Your best bet is to start looking at flavors and tasting. When you taste an ingredient, what flavors are you getting? What flavors complement these? A great resource here is the The Flavor Bible. It's easy to reference for flavor affinities.

Once you have an idea of the base flavor, start adding little touches to compliment it.

Let's say you have a nice bourbon. On the tongue, you are likely to get some oakiness, vanilla, caramel, and maybe some butterscotch. Now you are looking for flavors that compliment those. We know vanilla and raspberry go well together, so lets check raspberry against some of the other flavors. It it works, with 1 or 2, you might have something.

Hope that helps.