1) Print the following portrait image and cut it out:


2) When Yuffie joins your party, be sure to name her "Aeris". If Yuffie is already in your party and you didn't name her "Aeris", then you cannot revive her.

3) After Aeris dies, whenever Yuffie, now named "Aeris", is on-screen, put your Aeris potrait cut-out over her face and presto!, you've revived Aeris.