* "Correct to prepare" is correct if it's part of a larger phrase like "It's correct to prepare in advance" or "The instructions are correct to prepare the mixture this way." Here, "correct" acts as an adjective modifying the infinitive "to prepare."
* "Correct for preparing" is correct if "correct" is an adjective modifying a noun phrase referring to a method or process. For example, "These are the correct steps for preparing the recipe" or "This is the correct temperature for preparing the batter." Here, "correct" describes the "steps" or "temperature" as appropriate for the act of preparing.
In short, there's no single "always correct" answer. The correct usage depends entirely on the surrounding sentence.