This starts with the contract. So many questions that you'll have along the way can be answered while discussing the contract. Feel free to start with the offered contract, or with the PEN model contract, and work from there. Figure out WHY things are done the way they are. For example, if your contract is a work for hire contract, that actually means that you'll most likely have little to no say in the editing process, for better or for worse. But ask up front. "How involved will I be expected and allowed to be in the editing process?" Will I ever see my translation between the final draft and the published book?
Another thing: payment. Maybe you're going to get paid upon final delivery. Great. But is that payable on receipt? Is it Net 30? You'll feel better if you know when your money is coming in, and if you know it sooner rather than later.
Next, before you even start translating, talk to the editor. After you've read the book send her/him a list of stylistic questions. "How do you want me to handle the historical present tense?" "How much slang are you comfortable with this character using in English? There are equivalents to XYZ in the original."
Just ask. Ask your editor, the admin assistant, anyone who's involved. Don't be afraid of asking stupid questions, because chances are, they'll all be important at some point.
P.S. This was a tough love letter to myself. Dear Allison, I'm writing to you. Fix your mistakes. Learn from them. Do better next time. I know you can.