Step 1 :
Download a clean FireRed rom named "1636 - Pokemon Fire Red (U)(Squirrels).gba"
*Search it on Google or something*
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Step 2 :
Download an IPS patcher ( I personally use Lunar IPS )
*Again, you can search for it in Google*
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Step 3 :
Open your IPS patcher, select the Moemon IPS file ( the file you downloaded from here ; extracting is optimal ), then select the Pokemon FireRed rom to start patching.
*Getting duplicates of the FireRed rom and the Moemon IPS file for backup is advised so you can try it again if it (somehow) fails*
After patching, you should get the modified FireRed rom with Moemons in it.
*Renaming it is advised so you wouldn't get confused which is the Moemon rom and which is the clean FireRed rom.*
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Step 4
Open your VBA ( this emulator works best for GBA games ; download it if you haven't yet ; I personally use VBA Link though so I can trade Pokémons ) , go to Option, then Emulator, click Save Type, then select Automatic AND 128k Flash. This is done so that your save files won't be corrupted.
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Step 5
In your VBA, select File, choose Open, then navigate to where your Moemon rom (the modified FireRed) is saved.
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Your game should be working at this point, starting from selecting a gender, name, rival and starter Moemon. Enjoy and hope this helps.