Its not that hard. You read the project plan and start translating it into a list of todos. As you make the list, questions come up. Write those down. When you are done writing your todo list, you send your list of questions to the relevant person, then start working on the todo list where you don't have questions, or where you are decently sure what the answer will be. As you work through the implementation, you will come up with more questions for edge cases. Send these to the relevant person as well.
Ideally, these questions will be sent as an enumerated heirarchical list, and will be answered as such in turn. Every once in a while, it is more useful to jump on a call to figure something out quickly.