Thunderstore

Thunderstore is the site to find mods.
r2modman and Thunderstore Mod Manager are mod managers meant to be used to manage mods from Thunderstore. This site will refer to r2modman, but Thunderstore Mod Manager can be used with the same instructions.

If you would prefer a video tutorial, you can find one here by Jeditobiwan:

Installing Mods via r2modman

Once you have installed r2modman and opened it, you will be greeted by a page asking you which game you would like to manage. In our case, we want to manage H3VR so click on that. It will ask you to create a profile or select the default one. It's up to you which you use, the default one cannot be deleted, but comes as an empty profile for new users.

You will then be greeted to the main interface shown below. Every time you want to run the game with mods, you will need to press the Run Modded button in the top left corner. This will launch the game with the mods listed under the Installed section on the left.

r2modman contains a list of Thunderstore mods in their Online section. It will automatically download any mods required by any other mods. This means all you need to worry about is the mods you want, and not what's needed to run it. This process also works in reverse: if you were to uninstall a mod that other mods depend on, those mods will also be uninstalled.

From the Online section in r2modman, install anything you want by clicking on it, then by clicking on the Download button. Below is a picture of the WurstMod dependency as an example.

Installing mods not on Thunderstore

Please see the Importing External Mods doc for more information.

Troubleshooting

Many common problems can be found on the r2modman wiki page, Why aren't my mods working?, provided by Ebkr.
For less common problems, see the Troubleshooting articles for more details. New problems are found every day, but yours will likely be on this page.

Configuring Mods

Some mods may utilize configuration files to change certain aspects of their behavior. Using r2modman's Config Editor setting on the left, you can change any config file created.
To generate config files, fully load the game. This will generated config files for all the mods currently enabled. Errors may prevent options within the config files or even entire config files to not generate. If you do have errors, fix those before changing config options.

Below is a picture of where the Config Editor feature is.

Running your game with mods

To run mods, you must press the Run Modded button within r2modman.

If you would like to run the game modded through Steam, making the ease of use and usefulness of profiles irrelevant, please see the bottom of the General section of the Help menu on the left.