For Tifa:
Accept Tifa's drink [CHAPT 3]
Describe Tifa as being in great shape to Chocobo Sam [CHAPT 9]

For Aerith:
Ask Aerith "how much" for the flower [CHAPT 2]
Don't say "What does it matter what I think?" to Aerith when she asks about her outfit [CHAPT 9]
Tell Chocobo Sam that Tifa is "good with the books" [CHAPT 9]


Choosing the opposing options when choices are given helps for the opposite girl, but you SHOULDN'T worry much, because you can still be nice to both girls and get your preferred girl in the CHAPT 13 cut scene. If you have already messed up though, these should help.

SIMPLE EXPLANATION:
Be nice to who you want, be mean to who you don't.

In chapter 3, simply do all the side quests offered to you, and you will get an extra cut scene [Still CHAPT 3 - Home Sweet Slum] with Tifa, letting you choose her outfits for chapter 9, the WALL MARKET chapter.

Doing this will add points to tifa and set her up to be the cut scene in chapter 13. [The exclusive cut scene]

SIMPLE EXPLANATION:
Do all Chapter 3 side quests

Although if your going for Aerith, you can still do all the chapter 8 side quests, and the points between Tifa and Aerith will equalize, along with you having gained the extra content in those chapters.

SIMPLE EXPLANATION:
Do chapter 8 side quests

The final decision maker is in the VERY BEGINNING of chapter 10 [Rough Waters] will ultimately choose the girl, Cloud [you] ]will choose to "help" Aerith or Tifa. This is the final decision that goes towoard Tifa/Aerith.

KEEP IN MIND, you can still be nice and do all quests, and get the girl you want. The final choice is what caps the choice. Being mean just helps you to like screw up less.


SIMPLE EXPLANATION
All Tifa side quests = +1 for Tifa
Conversation choices for Tifa [against Aerith] = +2 for Tifa

All Aerith side quests = +1 for Aerith
Conversation choices for Aerith [against Tifa] = +2 for Aerith

Choice in chapter 10 = +1 for chosen girl

Essentially avoid all the extra side quests with the girls. This isn't recommended for the achievement grabber. Sorry Barret fans

SIMPLE EXPLANATION:
Do not do it. Girls are better anyway.