I've seen this trip up other developers as they make the transition to senior or lead. Here are a few suggestions I've made to others.
- Understand what the goal of the project is.
Often it isn't about all the features that have been pushed into the project. It's about a core set of functionality that is addressing a business need. Always keep this in mind because that is your primary goal.
- Breakdown what needs to get done into tasks. Understand the dependencies between them.
Breaking a project down should be pretty straightforward. Break it down as early in the project as you can. If you have to gloss over parts, understand that they pose a risk until you understand what has to be done.
- Understand what the open questions or ambiguities of the project are.
You won't be able to resolve all the ambiguities initially (although you should try). Make sure your manager and the project stakeholders understand what they are and what risks they pose to the project.
Make sure everyone knows (ideally on a daily basis, but weekly works) what the status of the project is. And by status I mean what has been done, what is left to do, open questions, problems, etc. Anything that can impact the completion of the project should be reported.
- Everyday, go over the big picture.
You should go over the big picture every day for an hour. Ask yourself the questions. What's been completed? What's left to do? What are the open questions? What's the goal? You should be able to give someone a detailed status of the project whenever they ask.