I recommend that you have very clearly defined budgets for the full project but absolutely avoid fixed bids.
Start with a small increment of a day or two and gradually increase the billing increments to a week or two. Don't continue work until you have gone over deliverables with the person who is "writing the checks" at the end of each stage (every few days at first). Agree on a fee for that upcoming period after discussing the work for that time frame. Make sure they are comfortable and if not stay with the smaller periods.
Start with a small increment of a day or two and gradually increase the billing increments to a week or two. Don't continue work until you have gone over deliverables with the person who is "writing the checks" at the end of each stage (every few days at first). Agree on a fee for that upcoming period after discussing the work for that time frame. Make sure they are comfortable and if not stay with the smaller periods.