The first step to all this is creating a folder to store the English patch, and other mods which may come out. To do this, you'll have to go into your game directory.
To do this, simply right-click "Kuro no Kiseki" in your STEAM library, hover your mouse over the 'Manage' section, and click the 'Browse Local Files'. It should look something like this:

After you've been brought to your game directory, you'll have to create your mods folder. This's literally as easy as creating a new folder, and calling it 'mods' or 'Mods' - the 'm' can either be lower or uppercase from my testing. At the end of it all, your game directory should now look like this:

Now it's onto the next step.

The next step is to go into your 'multilanguage' folder, and deleting two files called "message_jp", and "prompts_jp". Your 'multilanguage' folder' should look like this after your done:

Onto the next step.

Two Steps forwards and one step back. One day, likely sometime this decade, providing links to this stuff won't be that big a deal. I know somewhere right now an office worker is cringing because they knew they took away a very accessible way for people to get into this game now instead of in the future, which means NIS gets money today and not tomorrow. Providing the tools will have to be enough for now.
I'm gonna have to delete the Discord too it looks like, so I don't get taken to Flavor Town by NIS. The patch is out there if you wanna find it, really not too difficult.
Everything in this guide still holds true for instructions so I'll keep this guide up, hopefully it doesn't get taken down.

The next and final step is to simply put the "ENGPATCH.p3a" file into the 'mods' folder you created earlier. At the end of it all, your 'mods' folder should look like this:

And with that: you're done!
Have fun playing Kuro no Kiseki with the fan English translation. It's a really good game.

1.) Every single time the game is updated, you'll have to repatch the English translation in. Just follow the same instructions as before and it'll work just fine. Keep a copy of 'ENGPATCH.p3a' somewhere on your system for when this happens.
2.) This works just fine for the STEAM DECK. Instructions are the same.
3.) This's a FAN translation, don't expect everything to be perfect. A bunch of people took time out of their day to give us a way to play Kuro no Kiseki in English for free, be grateful we even have that and don't have to wait 2-3 years for NIS to catch up.

If you scroll down to the comments, you can see where I recommended you validate your files after the Official Patch drops. Turns out, I was wrong. You must completely uninstall the mod to play the official version. Not only that, but based off my testing, you can no longer even play the game with the Unofficial patch; at least not without zany means that even I, being brazen and autistic enough to even post this guide to Steam, am unwilling to detail.
You must do two things:
1.) Uninstall the 'Mods' folder you created for the Unofficial Patch.
2.) Set your game text language to Japanese within the game options. Remember, this mod patched OVER the Japanese text, your game is still set to Japanese.

Your game folder should look like this after you are done. Take not of all the extra files now in there after the Official Patch.

Here is how to switch your game language to English. Launch the game, and go to the 'System' Menu.

Where it says 'Text Language', Left-Click on it once. It will switch the text-language over to English.

You will have to restart your game for this to work. It'll look like this.

You should be good now, my testing indicated as much. I did not have to validate my files after I did this, but you should do it anyway.
The fact this guide got so much traction is a good sign. I hope NIS do this for Kuro II. That hope is why I'm not buying the Chinese Version of Kuro II, because modding NIS versions are much easier. The NIS Version is coming out early next year, and it looks like they're going to have a Day One English Localisation. We've gotten so much Kiseki over the past few years, it's hard to believe it was a relatively unknown brand in the West five years ago.