EDIT: version 2 -- segmented the instructions better. Now vanilla and OpenMW are totally separate and Optional mods are found in the end of each instruction set, where they hopefully don't overwhelm the reader.

The installation instructions provided in the latest readme file were outdated, due to the switch to the new BAIN folder structure. Also, so many people now are using OpenMW that it seems prudent to include instructions for it. Here is the rewritten section II:

====================================================
II INSTALLATION
====================================================

Tamriel Rebuilt can be installed on either the original Morrowind engine (vanilla) or OpenMW. Depending on your choice, follow the corresponding section below.

====================================================
IIa VANILLA MORROWIND
====================================================
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i. Requirements
----------------------------
Before installing Tamriel Rebuilt, please ensure you have installed the latest version of Morrowind (v 1.6820), including both Tribunal and Bloodmoon. You also need Tamriel_Data version 7.1 or later. The Morrowind Code Patch is recommended. No other plug-ins are required.

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ii. Install Process
----------------------------
Tamriel Rebuilt comes packaged in the BAIN format -- a folder structure designed to be automatically parsed by the mod managers. As such, you may choose to install the mod using Wrye Mash (https://www.nexusmods.com/morrowind/mods/45439/). However, beginners to Elder Scrolls modding may have an easier time installing manually.

To do so, simply open the downloaded .7z file using 7-zip, enter the directory '00 Core' and move/extract TR_Mainland.esm to your Morrowind 'Data Files' directiory. You will find this directory inside the main Morrowind install directory that contains the .exe file.

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iii. Registering Game BSA
----------------------------
If this is your first time installing Tamriel Rebuilt, you will need to register the BSA files that come with Tamriel_Data. Find the Morrowind.ini file in the game directory (not the 'Data Files' directory, but where the .exe file is) and open it in any text editor. Search for the [Archives] section of the INI, where you should see a list of registered game archives from the Tribunal and Bloodmoon expansions. You must add these lines below the list of archives:

Archive 2=PT_Data.bsa
Archive 3=TR_Data.bsa

with the Archive number being the next available number in the list. In most cases it will be 2. Once that is complete, save and close the file. You only need to do this once.

NOTE: If you are using Mod Organizer 2, these lines won't register. Instead, see the applicable section here: https://www.tamriel-rebuilt.org/about/frequently-asked-questions/bsa

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iv. Enabling Game Files
----------------------------
The final step before playing the new content is to ensure that the data files are enabled in Morrowind's Launcher. Simply launch the program and ensure the Tamriel_Data.esm and TR_Mainland.esm are ticked.

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v. Optional Files
----------------------------
You don't need the following files to play, but some are recommended. You can find them in other folders of the downloaded .7z file. To install, move them into your Morrowind 'Data Files' directory and enable them in the Morrowind Launcher.

RECOMMENDED:

- TR_Factions.esp (in '01 Faction Integration'): better integrates the Vvardenfell factions into the mainland factions.

- TR_Travels.esp (in '03 Travel Network for Core and Vvardenfell'): provides easy access to the mainland through standard travel providers on Vvardenfell.

OPTIONAL:

- TR_Preview.esp (in '02 Preview content'): adds work-in-progress areas that you can visit for a sneak peak at our upcoming content (you will not yet find npcs or quests in most of these areas). Use this with TR_Travels_(Preview_and_Mainland).esp (in '03 Travel Network for Core, Preview, and Vvardenfell') instead of TR_Travels.esp. Beware that this file will change drastically between Tamriel Rebuilt releases and may result in your saves being incompatible with future updates. As such, we recommend using a different save for this file.

- TR_OldTravels.esp (in '04 Patch for Abot's Travel Mod'): compatibility patch for Abot's Travels mod.

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v. Updating Save Games (if using existing save with a previous version of TR):
----------------------------
If you are updating Tamriel Rebuilt, and intend on resuming play using an existing save-game, you must first run the file patcher contained in Tamriel_Data download and then undertake the additional steps outlined here (http://wiki.theassimilationlab.com/mmw/Wrye_Mash#Using_an_Updated_Versio...) to resolve any potential errors when using an old save-game on a newer version of Tamriel Rebuilt. Please note, you will need to download and install the Wyre Mash utility to clean and repair your savegame using the method above as well as a Java Runtime Engine to use the file patcher.

====================================================
IIb OPENMW
====================================================

----------------------------
i. Requirements
----------------------------
Before installing Tamriel Rebuilt, please ensure you have the latest version of OpenMW with a working instance of Morrowind, Tribunal and Bloodmoon. Tamriel Rebuilt requires Tamriel_Data version 7.1 or later. No other plug-ins are required.

----------------------------
ii. Install Process
----------------------------
Tamriel Rebuilt comes packaged in the BAIN format -- a folder structure designed to be automatically parsed by the mod manager Wrye Mash. However, support for OpenMW is still work-in-progress for that program. Instead, install the mod manually:

1. Create a new directory for Tamriel Rebuilt somewhere on your computer.
2. Open the downloaded .7z file using 7-zip.
3. Enter the directory '00 Core' and move/extract TR_Mainland.esm to the directory you just made.
4. You need to register the folders of both Tamriel Data and Tamriel Rebuilt inside openmw.cfg. Locate this file at $HOME/.config/openmw (Linux), $HOME/Library/Preferences/openmw (MacOS), or C:\Users\<USERNAME>\Documents\My Games\OpenMW (Windows).
5. Search the file for the last line containing 'data=', and add the following lines below:

data="<PATH>Tamriel_Data"
data="<PATH>Tamriel Rebuilt"

replacing '<PATH>' with the filepath where your Tamriel_Data and Tamriel Rebuilt folders are located. The double quotes are mandatory.
6. Save the openmw.cfg file.

(Alternatively, you might be interested in trying Mod Organizer 2, which works with OpenMW using the plugin here: https://www.nexusmods.com/morrowind/mods/45642/)

----------------------------
iii. Registering Game BSA
----------------------------
If this is your first time, you will need to register the BSA files that come with Tamriel_Data. To do so, locate your openmw.cfg file (see above), search for the last line containing 'fallback-archive=' and add these lines below:

fallback-archive=TR_Data.bsa
fallback-archive=PT_Data.bsa

Save and close the openmw.cfg file.

----------------------------
iv. Enabling Game Files
----------------------------
The final step before playing the new content is to ensure that the data files are enabled in OpenMW's Launcher. Simply launch the program, navigate to the 'Data Files' tab and ensure Tamriel_Data.esm and TR_Mainland.esm are ticked.

----------------------------
v. Optional Files
----------------------------
You don't need the following files to play, but some are recommended. You can find them in other folders of the downloaded .7z file. To install, move them into the Tamriel Rebuilt directory you made and enable them in the OpenMW Launcher.

RECOMMENDED:

- TR_Factions.esp (in '01 Faction Integration'): better integrates the Vvardenfell factions into the mainland factions.

- TR_Travels.esp (in '03 Travel Network for Core and Vvardenfell'): provides easy access to the mainland through standard travel providers on Vvardenfell.

OPTIONAL:

- TR_Preview.esp (in '02 Preview content'): adds work-in-progress areas that you can visit for a sneak peak at our upcoming content (you will not yet find npcs or quests in most of these areas). Use this with TR_Travels_(Preview_and_Mainland).esp (in '03 Travel Network for Core, Preview, and Vvardenfell') instead of TR_Travels.esp. Beware that this file will change drastically between Tamriel Rebuilt releases and may result in your saves being incompatible with future updates. As such, we recommend using a different save for this file.

- TR_OldTravels.esp (in '04 Patch for Abot's Travel Mod'): compatibility patch for Abot's Travels mod.