This repository is a combination of small routines which shows the category icon for a move under the Physical Special Split in relevant UIs: the TM Case's info section, the Move Relearner's UI, a Pokémon's move summary screen, and in battle.
Obviously, this hack assumes that you have inserted the Physical-Special-Status split, and set the category for each move.
Update your ROM's types image to have the new icons in the gfx.png image file.
You can compare it with gfx_original.png to see what has been added.
You can overwrite the image at the following address with a tool such as GBA Graphics Editor.
Types image graphics address: 0xE95DDC
Types image palette address: 0xE95DBC
Provided you do not make the new file any larger, repointing of the image will not be necessary.
Update your ROM's types image table to support the new icons.
Types image table address: 0x452C94
Assuming your types image exactly matches gfx.png, you can simply insert type_table.bin into your ROM and repoint from 0x452C94 to wherever you insert it.
For more complex adjustments to the types image and its table, refer to the PokéCommunity tutorial on adding new types to FireRed for an indepth explanation of how the table and image are linked.
You may also need to update CATEGORY_ICONS in config.s to make it match the index for the category icons in your ROM's own table.
First, open config.s in a text editor to customize it.
Tweak the definition of free_space to be where you want the code inserted.
Update move_data and move_descriptions as needed.
This hack also supports Zeturic's Unhidden Power.
If you wish to enable this support, change unhidden-power from false to true in config.s.
Then, set the label hp_type_decode_addr to the address (NOT +1) of the routine with the same name from the Unhidden Power repository.
You'll need to have armips.
Place your ROM in this folder and name it rom.gba.
Once you're ready, assemble with armips split-icons.asm; it'll generate test.gba, which is your output (rom.gba will not be modified).