EDIT: this guide is now updated and will show you:
-How to install OpenFusion 1.4
-Troubleshooting
-How to use the commands
-More Info
-How to play FusionFall offline.

Installation:

1) Download the latest version of OpenFusion, either the portable (zip) or install (exe) version.
I just used the portable one:
https://github.com/OpenFusionProject/OpenFusion/releases

For the portable one, right click the zip file, (with 7-Zip for example) and extract it to where you want.
You can make a shortcut to the file "OpenFusionClient.exe" to your desktop
and open it.

2) Select a version/server and start the game.
Make an account by simply typing a username and password
into the OpenFusion FusionFall login page (don't click Register, just log in
as if you already made the account, that's how it seems to work).
It won't work if the username has already been taken.
Remember your login details.
I have no idea if your password is visible to anyone (or vulnerable to hackers) so just be
aware and don't use a password you use for other things.

EDIT: I think accounts are wiped after every update but it's no big deal
starting again due to all the cool commands you can use.

Troubleshooting:

The latest version is easier to setup but you might get an error
if you haven't installed the following:
Microsoft .NET Framework 4.8
(if you need it, the download link for the offline installer is under
"Download information" within this page:
https://support.microsoft.com/en-us/topic/microsoft-net-framework-4-8-offline-installer-for-windows-9d23f658-3b97-68ab-d013-aa3c3e7495e0)

If you get an error trying to install .NET Framework 4.8,
you probably need the following Windows updates first which you can install manually:
KB2813430
KB3033929
Links for Windows 7:
https://www.microsoft.com/en-us/download/details.aspx?id=39110
and
https://www.microsoft.com/en-us/download/details.aspx?id=46078

After installing these two windows updates (PC restart may be required?)
you can now install Microsoft .NET Framework 4.8 successfully,
now allowing you to run OpenFusion!

What if you get a Unity Web Player "couldn't load plugin" error?
Try installing Unity Web Player by going to the "installUnity.bat" file (inside the "utils" folder),
right click > run as admin.
Then also open "UnityWebPlayer.exe" as admin.
If it's still not fixed, you may get the "Failed to update Unity Web Player" error when trying to run the game.
If so, try this:
Inside the "utils" folder is a folder called "WebPlayer".
just copy the file contents that's inside the WebPlayer folder and paste it into
C:\Users\(your username)\AppData\LocalLow\Unity\WebPlayer

If that doesn't work, try uninstalling unity web player before pasting.
If nothing works, try both the install and portable version of OpenFusion if you haven't already.

Using the Commands (Examples)

Type into the ALL chat box to try out these commands:

/speed 3000 (or try 5000 to travel around the map faster)

/jump 15 (or try 30 for huge jumps)

/nano 36 (this is to level up to max level)

Entered speed/jump commands tend to expire after shooting or receiving damage
so have to be re-entered.

For items:
/itemN [type] [itemId] [amount]

So for example, if I wanted the Action Blindfold,
I would enter:

/itemN 5 11 1

Why? Because /itemN (Face5) (ID 11) (1 copy)
Go to the item list on that github page: https://docs.google.com/spreadsheets/d/1mpoJ9iTHl_xLI4wQ_9UvIDYNcsDYscdkyaGizs43TCg/edit

(You can also download that document as a PDF for quicker access).

Use CTRL + F to search for an item, I searched for Action Blindfold,
it has these coordinates:
"Face5" and "ID 11".
So enter into the All chat:
/itemN 5 11 1



If you wanted 2 copies of that item, then it would be /itemN 5 11 2

For more information including commands, read that github page
https://github.com/OpenFusionProject/OpenFusion
(like private servers and stuff).

Commands List:
https://github.com/OpenFusionProject/OpenFusion/wiki/Ingame-Command-list


More Info:

You can see who is online with this website!:
https://dexlabs.systems/ (Original server)
https://academy.dexlabs.systems/ (Academy server)

You can also check out Retrobution:
https://github.com/JadeShrineMaiden/OpenFusion/releases/tag/1.3

How to Play FusionFall Offline when you don't have internet:

You can play FusionFall offline using BlueMaxima's Flashpoint.
But another way is to use OpenFusion and download the server zip files
"OpenFusionServer-1.4-Academy.zip"
"OpenFusionServer-1.4-Original.zip"
from here:
https://github.com/OpenFusionProject/OpenFusion/releases

You can extract each server zip file into the OpenFusion 1.4 folder if you want.
Then if you wanted to run the original server offline, go to
the "OpenFusionServer-1.4-Original" folder you just extracted
and open the file "winfusion.exe" (think of this file as a way to trick FusionFall into thinking you're online so you can login).
Now open OpenFusion and create a new server,
you can leave it all blank and name it something like OFFLINE.
You will be able to log in/make an offline account that will work on this
offline server even when your internet is off!
For your convenience, you can somewhat automate this with a batch file
so it will open both exe files for you:

Batch File:

1) Copy what's in this Code box and paste it into Notepad
(but replace the folder location of the two exe files to where you actually have them on your computer)

Code:

START "winfusion" /D "C:\Users\(your username)\Downloads\OpenFusionClient-1.4\OpenFusionServer-1.4-Original\" winfusion.exe

START "OpenFusion" /D "C:\Users\(your username)\Downloads\OpenFusionClient-1.4\" OpenFusionClient.exe

If you're not sure, right click your exe file > Properties > copy the part next to Location.
And make sure there are speech marks ("") like in the code above.

To be clear, the parts in blue are what you change:

Note that you still need internet to download the game files when OpenFusion first starts but once The Future
and The Past are fully loaded, you could just keep a backup copy of the FusionFall cache files if you want to guarantee offline play.
(The cache folder location is C:\Users\your username\AppData\LocalLow\Unity\Web Player\Cache)

On Notepad save this file and name it what you want followed by .bat (not .txt) e.g. FusionFall.bat

2) It will work if you did it correctly.
Now you can create a shortcut of this .bat batch file and place it on your desktop.
Rename the shortcut, e.g. to FusionFall Offline or whatever you prefer.
Now right click the shortcut > Properties > Shortcut tab > Change icon > now browse for the OpenFusionClient.exe file
to use its shortcut icon. Done.
Opening this shortcut will open both the server file (winfusion.exe) and then OpenFusionClient.exe for you.
Then just go into the offline server you made earlier and login.

EDIT: I put the following older guide in spoilers as it's outdated.

Outdated Guide:

Been trying to figure out how to make this thing work and this Madlad actually did instead. 

Undelos, where have you been? I guess life gets in the way.
This guide was outdated for a long while (over 2 years??  ) but I've finally updated it
for OpenFusion 1.4
This is cool, you can see who is online (and where on the map) with this website:
Let the stalking begin! jk.
https://dexlabs.systems/ (Original server)

https://academy.dexlabs.systems/ (Academy server)



Finally got this version to work on my PC.
Only thing missing is, it keeps forgetting my login.
And the buddy list system doesn't seem to work
but the above is a decent way for now to see who's online and where. 
You can then warp to someone by using the commands.

https://github.com/OpenFusionProject/OpenFusion/wiki/Ingame-Command-list

I'm a bit surprised so few people are usually online.
There were about 2 other people online on the main server last I checked
(EDIT: on different times of the day, this may change, I'm now seeing 9 people online).
Maybe most players aren't aware of OpenFusion, got errors running it
or don't have the time to play these days.
Still, it's great we can play FusionFall again.

I forgot about Retrobution, apparently the buddy list works on there (not working for me ).
I also used to get an error but was probably fixed with the .NET Framework 4.8 thing then I got
a white screen, fixed by allowing Retrobution through my super high security firewall.
Link:
https://github.com/JadeShrineMaiden/OpenFusion/releases/tag/1.3

Might also look into how to get a vehicle and make a private server with Halloween events and stuff.
Or maybe make an FF movie....and actually finish it this time.
Oliver: stop lying to yourself.
*a decade later*
Finished 7 Legends yet?
X: almost

X... I think the buddy system doesn't work for you cause none of us were online at the time

But wouldn't it say you're offline just like in the original game,
instead leaving me hanging and doing nothing (except the sound effect of having entered something).
Not even "this user doesn't exist" for made up names
or "this user is offline" or anything.
I guess that part hasn't yet been implemented into OpenFusion??  
Wait, are you telling me, that the buddy system works flawlessly for you, Oli
exactly like the original FF?
If that's the case, then it's a problem on my end.
In OpenFusion your character name is Oli Zombieweasel right?
I also tried Oliver Zombieweasel, Rex Undelos, Hom Choi....
It doesn't do anything.
The only time something comes up is if you type
a name without a surname, it will say to enter a first and last name.
Same thing in Retrobution.

Btw, you can add FF codes in Retrobution but not sure how on OpenFusion.
I instead just managed to get the Knishmas Kimchi Cloud vehicle by finding the item code in the spreadsheet I linked a while ago
and entering this command in OpenFusion:
/itemN 10 17 1

Does OpenFusion and Retrobution forget your login details every single time too
or is it just me?

XLR8 147 wrote:

But wouldn't it say you're offline just like in the original game,
instead leaving me hanging and doing nothing (except the sound effect of having entered something).
Not even "this user doesn't exist" for made up names
or "this user is offline" or anything.

X... that's how it worked in the original game.

XLR8 147 wrote:

Does OpenFusion and Retrobution forget your login details every single time too
or is it just me?

Yeahhh that's everyone. The profile is saved, but you still have to enter your details each time

Oli Zombieweasel wrote:

X... that's how it worked in the original game.  

I wonder, which of us is misremembering the game?
Find out in the next episode of XFF Legends.
I'm sure in the original game if you tried to add someone offline to your buddylist
it would actually say they're offline. You don't remember?
And if you tried to add a made up username (or one with a typo),
it would say the username doesn't exist.
You tellin' me I dreamt this whole thing up?
BRB suing Oli for making me think I made it all up. 
All your jawbreakers are belong to us!

If only I could find evidence on an old FF youtube video.
Fine, you win this time. 
I'm sure you just forgot though, it's been a while since the original FF shut down (2013).  
Original FusionFall last seen: 10 years ago
Undelos last seen: 1 year ago.
Hom Choi last seen: 3 years ago.
Dear Time, please stop flying.

Undelos says stuff all the time.
Hom, not so much...