Instructions
Select positive interactions with Tifa to raise your affection score. For example, when you return to the bar at the beginning of the game, choose to give your flower to Tifa instead of Marlene.
Talk to Tifa whenever you have a chance to speak to your party members. Choose to speak to Tifa before talking to anyone else.
Choose Tifa for your party whenever you are asked to split up. This not only increase your affection score with Tifa in some cases, it gives you more opportunities to have positive interactions with her.
Select positive interactions with non-playable characters when Tifa is in your party. For example, when at Fort Condor, select "I guess so" instead of "not interested" when speaking to the old man.
Select negative interactions when speaking with Aeris, Yuffie or Barret. Likewise, try to ignore these characters or keep them out of the party whenever possible to avoid accidentally raising your affection score with them. If you must have them in your party and Tifa is not present, select negative interactions with non-playable characters to lower your affection score.