How to install a MyBB plugin (and how to undo it cleanly)

MyBB plugins are small: usually one PHP file, sometimes a folder, sometimes a language file alongside. The whole job is four steps and one undo.

Before you start

Back up the database and inc/ — the ten-minute version.
Check the release checksum — how.
Note your MyBB version. Plugins are written for a major version; 1.8 and 1.9 are not the same target.

1. Upload the files

Connect over SFTP or use your host's file manager and go to your forum root, then inc/plugins/. Upload the plugin file, or the folder, exactly as it came out of the zip:

/inc/plugins/your_plugin.php
/inc/plugins/your_plugin/        (only if the release has a folder)

If the release includes a language file, it goes to inc/languages/english/ (or the matching language folder) using the same relative path as the zip. Admin-side files, when present, go under admin/modules/.

Permissions: 644 for files, 755 for folders. That is what MyBB expects and what it will keep working with.

2. Activate it

Log into your board's Admin CP and open Configuration → Plugins. Your plugin is listed as inactive. Click Install & Activate.

This is the moment a plugin is allowed to create its tables and settings. If the button errors, the plugin is almost always built for a different MyBB version — do not force it by editing the file.

3. Set your options

Configuration → Settings → find the group named after the plugin. Set the options, save, then open the front end of the forum and confirm the behaviour you expected. Admin CP settings that never reach the front end usually mean the plugin was activated but its cache has not rebuilt — deactivate and reactivate once.

4. Wire it into your theme (only if the readme asks)

Template edits live in Templates & Style → Templates → your theme. Change the theme, never the master templates, and change a copy you can compare against — MyBB's template editor keeps a diff, so use it.

Undoing it, properly

There are two different actions and they are not interchangeable:

Deactivate — the plugin stops loading, its settings and tables stay. Use this to test.
Uninstall — the plugin's own uninstall routine runs and may drop its tables and settings. Use this when you are finished with it. Read the readme first: some plugins keep your data on purpose, some do not.

Then delete the files you uploaded. If anything looks wrong after that, put the pre-install inc/ folder back and restore the database dump.

The three failures you will actually meet

“Please upload a valid plugin” — the file landed outside inc/plugins/, or it was renamed.
White page after activation — PHP version mismatch or a missing companion file. Check error_log in the forum root before you guess.
Plugin active but nothing changes — a theme or template edit from the readme was skipped, or the plugin is setting-based and its defaults are off.