Start with the decisions that affect everyone
Record the destination, dates and travellers first. Add confirmed flights, accommodation and booked activities next, so the important facts are settled before the group starts filling every hour.
Give the itinerary a single home
Put each booking and idea into the day it belongs to. A shared timeline makes gaps, clashes and travel time easier to see, while keeping the latest version available to the whole group.
Keep preparation visible
Use shared packing lists and a combined budget alongside the itinerary. Everyone can see what is still outstanding without sending another reminder to the group chat.