Write it down
Growing up, I enjoyed listening to Jerry Clower and often quote him to this day. One of the stories I love involves an old married couple where the husband offered to get his wife a bowl of ice cream. She said yes and asked her husband to write down what she wanted so he wouldn't forget. He insisted he could "remember from here to the kitchen" and refused to write it down. After some time, the husband returns from the kitchen with a plate of scrambled eggs and his wife says, "I told you to write it down or you'd forget my bacon."
That's a cute story but this is a technology blog, what's your point? The point is, keeping people aligned and working in the same direction is difficult and writing things down helps reduce the difficulty.
Exponential Complexity
Most of us have probably seen the diagram that shows as you add more members to a team the number of lines of communication grows exponentially. That concept is based on the Complete Graph, which is a graph where every node is connected by an edge. The underlying assumption is that each member of the team must communicate directly with each other member of the team to ensure alignment. For an 8 member team, this translates to 28 lines of communication. When we write things down we reduce the needed lines of communication to 8 because we only require each member of the team to "communicate" with the written source of truth.
Write What?
To effectively reduce the needed lines of communication, your written source of truth must answer the questions asked most often by the team. Each team and project are different so the questions asked will be different, but at a high level, you need to answer where the team is going, why they are going there, how they will get there, and what it will look like when they finally get there. Along the way, you will necessarily iterate and refine your plans through a series of decisions, which also need to be documented.
Discipline is Key
Managing a single source of truth requires discipline on the part of leadership to ensure decisions are documented and plans are updated as required. It also requires discipline from the team members to regularly consult the source of truth and to remind their fellow team members to do the same. Finally, the team needs to speak up when the source of truth is missing information or when decisions are not clearly documented. If the source of truth doesn't stay up-to-date the team will need to open more and more lines of communication amongst themselves, reintroducing the communication complexity.
Final Thoughts
Accomplishing work as a team is complex and anything worth doing often takes much time effort. Make sure you write down your plan, keep it updated, and constantly refer back to it or in the midst of getting ice cream, you'll forget the bacon.