Define the core promise
Write one sentence: “This product helps [person] accomplish [task] without [current frustration].” If that sentence is unclear, a longer feature list will not help.
Separate “must work” from “nice to have”
User accounts, payment systems, matching algorithms, messages and dashboards may each represent substantial work. Some are essential to your core loop; others are future improvements.
Prototype the risky part first
For a marketplace, test whether both sides can complete the transaction. For a game, test whether the controls and progression are satisfying. For a business platform, test the workflow the team repeats every day.
Budget for more than coding
Allow room for design, testing, infrastructure, integrations, security, content, launch and maintenance. A quote without those assumptions is not a complete plan.
Choose a milestone you can evaluate
A good first phase ends with something reviewable—a prototype, validated flow or narrow working release—along with a clear decision about what should happen next.