How do I enter a contract with an Author in order to sell a book idea to them?

Simply right up a contract or letter of agreement with the terms you like--it's that simple. There's no magical or special language that has to be included. Just be sure to think about all the different scenariors or possibilities and try to cover you bases as thoroughly as possible.

However, before proposing the idea, write up a short non-disclosure agreement (you can probably find some samples by doing a web search) indicating that the prospective author may not use the idea you are disclosing to him or her other than in a book project with you. That way, you can disclose the idea in confidence to them, w/out them taking the idea and then running with it w/o you.