This page discusses how to install the modpack using Technic or MultiMC, and how to migrate the modpack from one installation type to another in SP. See Server Setup for how to setup a server for MP.

Installing

Technic

Download, install and start the Technic launcher
Go to the Modpacks tab and enter "GT New Horizons" in the search box. Make sure the pack named "GT New Horizons" is selected in the results list.
Optionally click the Modpack settings link in the upper right to select a specific version and/or install path. By default, the latest recommended release will be installed, but some servers may use other versions.
Click the Install button on the lower right to install the pack.
Before launching the game, you may need to adjust your Java settings. Go to the Launcher settings at the top right corner, and the Java tab of that dialog. At least 4 GB is recommended. Make sure you are using an up to date, 64-bit Java 8 (!) version.
Click Play to start the game.

MultiMC

GTNH can easily be installed on MultiMC by adding a new instance and importing the ZIP archive available from http://downloads.gtnewhorizons.com/Multi_mc_downloads/.

Note that the above only applies to stable releases (eg 2.1.X.0). If you would like to install a new dev release, you must first install a stable release, then follow the steps in XYZ to upgrade to the dev release.

For old dev builds (like versions prior to 2.0.8.3), you will need to prepare a vanilla instance and install Forge.

First download MultiMC. It will be a simple zip file, so you will want to unpack it in your HD someplace - with new versions of Windows, your Documents folder may be backed up via OneDrive, so you might want to/not want to install there.
Next, go to Add Instance and choose 1.7.10 in the Vanilla tab. Give the instance a meaningful name.
Select your new instance and click Edit Instance. There click Install Forge in the Version tab. The required Forge version is 10.13.4.1614 (which is the latest and should be the top entry in the list).
Now you need to get the modpack files. These are easiest to get from http://downloads.gtnewhorizons.com/ClientPacks. Make sure to download the correct version.
In the zip archive you will find a minecraft folder, that contains all the important files. You will need to copy its contents to the .minecraft folder of your MMC instance. You can jump to the appropriate folder from within MMC by selecting the instance and clicking the Minecraft Folder button on the right.
If you feel fancy, you can also grab the modpack logo 42x42Icon.png from the zip file. Then go to MMC, right click your instance, choose Change icon and use the Add Icon button.
If you are playing locally, backup the journeymap, tcnode tracker, and save directories.
If you are playing remotely, you still need to backup the journeymap and tcnode tracker directories.

Curseforge Launcher

Not recommended at this time due to their new license requirements on mods. And the fact that it constantly messes up recipes and is generally out-of-date. Please choose a different option.

Upgrading

Technic

Recommended releases are pushed to Technic at a slower rate, once they have completed testing. Sometimes releases can be pushed to Technic for testing, to see those look in the Technic configuration window.

MultiMC

Assuming you have storage, first backup your existing GTNH instance with right click -> Copy Instance. (If you don't have a GTNH instance yet, refer to Installing above.) This will help make sure you still have access to settings / configs for reference, and don't lose your screenshots, keybinds, etc.
Delete the config, mods, resources and scripts folders from your instances .minecraft folder.
Copy in the folders with the same names from the downloaded archive.
Port over any customized config settings (this is where your backup comes in handy for reference).
Update your questbook. You can either use the default load block, go to cheat mode and do /bq_admin default load to load quest book changes, or do bq_admin default load on a server.

Migrating to a different installer

This guide will go over migrating your installation to a new launcher.

Files to backup

If playing in single-player, backup your "saves" folder.
The journeymap folder.
The TCNodeTracker folder
Any custom resources/resourcepacks you are using.
options.txt (also optionsof.txt if you use optifine).
Your screenshots folder, if you want to keep them
Any custom (client-side or not) mods such as InGameInfoXML and its configuration file.
Your backups in the backups folder (feel free to only grab 1 or 2 if you aren't deleting it)
Any custom changes to the config files or script files
Consider keeping the logs folder if you crash a lot