I recommend:
- Demonstrate your application to the customer often, and get feedback early.
- Break stuff down into small tasks that you are very confident about estimating time for. This is key.
- Focus on minimal functionality first - login, logout, visit home page, etc. before the real features.
- Use an Agile methodology.
- Talk to your client daily.
- Use software that you, the client and programmer can share for tracking everything (features, bugs and Chores) such as Pivotal Tracker(which is my favorite), Trello, Jira or MS Project.
- Make sure you have a contract.