A wish list of cities, a rough timeframe and a budget range is enough. Three things genuinely improve the result: which cities are must-haves versus nice-to-haves, how flexible your dates are, and why you are taking the trip. You do not need to know fare classes or routing rules.
You do not need a finished itinerary. A wish list of cities, a rough timeframe, a budget range and any fixed constraints is enough to start. Some people send a spreadsheet of 14 cities with exact dates. Others send two sentences. Both work.
Three things genuinely change the quality of what we build. First, tell us your must-haves versus your nice-to-haves. If Machu Picchu is the whole reason for the trip, it anchors the routing. A "maybe Greece?" gets included only if the routing and pricing support it.
Second, tell us about flexibility. Even a few days of movement on a date can save hundreds of dollars on a specific leg. If you must be in Sydney on December 20th we will build around it, but if "sometime around Christmas" is close enough we can shop a much wider window.
Third, tell us why you are going. It sounds soft and it is entirely practical. A honeymoon prices differently from a family trip with three kids under ten. The why shapes the pace, the layover tolerance and where the cabin class money goes.
What you do not need: fare classes, alliance rules, mileage tiers or routing logic. That is our job.