1. Create a text file in your stellaris root directory, "stellaris.exe" should be in the same folder. Name the file "flag".

2. Paste this into the text file.

change_country_flag = {

}

3. Start Stellaris, create a custom empire with the flag you want, save it. We just want to extract the flag data here, everything else doesn't matter.

4. Go to Documents\Paradox Interactive\Stellaris\user_empire_designs_v3.4.txt

5. Find the empire you've just created(you can search by name), copy its "empire_flag" section. It should look something like this

icon={
category="necroid"
file="necroid_10.dds"
}
background={
category="backgrounds"
file="flag_BG_27.dds"
}
colors={
"shadow_purple"
"black"
"turquoise"
"null"
}

6. Paste it inside the curly brackets in the "flag.txt" file. Be careful with the amount of brackets. The file should now look something like this

change_country_flag = {
icon={
category="necroid"
file="necroid_10.dds"
}
background={
category="backgrounds"
file="flag_BG_27.dds"
}
colors={
"shadow_purple"
"black"
"turquoise"
"null"
}
}

7. Load the game, enter the diplomacy screen with the empire you want to change the flag of(otherwise you will be changing your own flag), open console by pressing "~"(or "`") button, paste the following

effect_file flag.txt

press Enter.

The flag should now be changed.