I suggest billing per half day, pay as you go, and continuous delivery. You go over your progress and get paid, then plan for the next few days or week. Do not

I suggest billing per half day, pay as you go, and continuous delivery. You go over your progress and get paid, then plan for the next few days or week. Do not go for more than a few days or week without showing progress and getting paid and discussing with the client. Initially progress is just writing down specs or mockups or whatever's but within a relatively short time it needs to be some type of working system. So they should be able to see straight into the development process as its going. And since you get paid after a few days or week, your risk is also reduced.