Start with the basics
Check whether the game has a documented two-player version. A game designed for a larger group may need different dealing, turn order or scoring when only two people play.
Put it into practice
Read the equipment list and choose a session length that suits both players. A short demonstration is useful before committing to a full game.
Before your next session
Agree on rules and explain how a round ends. If you use a house variation, record it clearly. The best choice for your group depends on what both people enjoy, not a universal ranking.