Each phase ends at a gate
A gate is a decision that, once made, is expensive to reverse: the structure, the design direction, the content, the launch. For each one, write down who decides, what they need to see to decide, and the date by which they will. If any of those three is unknown, the phase has no end, and the timeline after it is fiction. The plan below gives the phase, its outputs, its gate, the usual owner, and a typical duration. Copy it into whatever tool you use and replace the durations with dates.

