1. Add the manifest https://raw.githubusercontent.com/Namo2/InPlayerEpisodePreview/master/manifest.json as a Jellyfin plugin repository to your server.
2. Install the plugin InPlayerEpisodePreview from the repository.
3. Restart the Jellyfin server.

Deprecated with JMP Version 1.11.0

Because the new JMP client is using the current web player from the server itself, it is no longer needed to make any changes to the client code directly.

This is the recommended way to install the script on the desktop client. If you don't feel comfortable editing the nativeshell.js file yourself (step 3 to 6), you can download the full release instead, which includes the script already added to the nativeshell.js file. It is yet unclear if there could be potential issues, replacing the nativeshell.js file with the one from the release, so it is recommended to follow all steps below.

1. Download the latest release JMP or JMP-full (includes the script already added to the nativeshell.js file)
2. Extract the zip file into your Jellyfin directory (e.g. C:\Program Files\Jellyfin\Jellyfin Media Player)
3. Inside your Jellyfin directory follow the folder path "web-client\extension"
4. Open the "nativeshell.js" file in a text editor.
5. Inside the file find the section const plugins = [];. Add a new line at the start of the list and paste in 'inPlayerEpisodePreviewPlugin',. The section should now look similar to this:

const plugins = [
    'inPlayerEpisodePreviewPlugin',
    'mpvVideoPlayer',
    'mpvAudioPlayer',
    'jmpInputPlugin',
    'jmpUpdatePlugin',
    'jellyscrubPlugin',
    'skipIntroPlugin'
];

6. Save the file and restart the JMP client.

1.1 Avoid this issue by using the file-transformation plugin.

1.2 Change Ownership inside a docker container

If you're running jellyfin in a docker container, you can change the ownership with thie following command (replace jellyfin with your containername, user and group with the user and group of your container):

docker exec -it --user root jellyfin chown user:group /jellyfin/jellyfin-web/index.html && docker restart jellyfin

You can run this as a cron job on system startup. (Thanks to muisje for helping with this solution)

1.3 Change Ownership running on a Windows installation

1. Navigate to: C:\Program Files\Jellyfin\Server\jellyfin-web\
2. Right-click on index.html → Properties → Security tab → Click on Edit
3. Select your user from the list and check the Write permission box.
4. Restart both the server and client. (Thanks to xeuc for this solution)

If none of the above solutions work, please have a look at old issues. E.g. here or here.