Wiki Tutorial Here (FireRed & Emerald)

Credits:

YeahPotato

For FireRed

Step 1

Navigate to Oak's Lab in HMA's map editor

Step 2

Double-click on one of the Starter Balls to open up its script

Step 3

Inside the script, you will need to edit some things:

• Change the number after setvar temp2 to the index of the Pokemon you want inside this Poke Ball

• Change the number after setvar temp3 to the index of the Pokemon you wish for the rival to receive if the player picks this Poke Ball

Step 4

Repeat Step 3 for each of the Poke Balls

Step 5

It looks like the rival gets a Pokemon from one of these balls, but really his teams in battle are pre-determined in data.trainers.stats, so you need to edit them. The rival will appear as Terry in the trainer table. Terry, Terry~2, and Terry~3 are the specific trainers used for the rival teams in the lab, but you will want to edit all of Terry's teams throughout the game to reflect your updated starters.

Happy Hacking!