First: It's difficult to advice on these matters in cyberspace. But as I have been in similar situations, I can give you some general advice: First, break thing

First: It's difficult to advice on these matters in cyberspace. But as I have been in similar situations, I can give you some general advice: First, break things up in smaller phases or sprints, setting some milestones based on sensible estimates. Get the team to commit to these estimates and the timeline. Second, ask the customer/PO to prioritize features (as others have pointed out) in Need to have, nice to have etc. Third, make the customer/PO prioritze what is most important in the project triangle: Time, scope, quality or price. If you make a simple plan (eg. a burn down graph), you can follow the progress and adjust the plan if necessary. You might consider doing a simple risk analysis as well. Good luck.