Threat modeling sounds heavyweight, but at its core it is four questions a team can answer in an hour: what are we building, what can go wrong, what will we do about it, and did we do a good job?
Draw the system
Sketch how data flows between users, services, and stores. Trust boundaries — the lines data crosses — are where most interesting threats live.
Prioritize ruthlessly
You will find more potential issues than you can fix. Rank them by likelihood and impact, address the top few now, and write the rest down for later.
Done early and kept light, threat modeling catches design flaws that are far cheaper to fix on a whiteboard than in production.