What you'll need:
Visual Boy Advance
Tile Molester (Or TLP or TileEd)
Hex Editor (Anything with a search function, XVI32 is a basic freeware hex editor)
A Rom (This tutorial uses a Fire Red rom)
For tools, Google is your friend.
I've written this tutorial for 12 steps and about 9 of them cover just finding the box. Okay, I'll get started now.
Step 1. Open up your rom in VBA and run it to a screen that shows the textbox. Avoid using the Oak Intro, as the following steps do not work with it. This is the place I used.
And I hope you learnt something today. Visual Boy Advance is a tool aswell. Don't take it for granted.