Select the Joomla 2.5 plugin which you plan to install. You can search for one in the Joomla extensions directory:
Make sure that it is compatible with Joomla 2.5.
Download it on your local computer and log in your Joomla 2.5 admin area. Navigate to Extensions -> Extension Manager. Click on the Browse button, find the plugin archive file and click on Upload & Install.
Once the plugin is installed you can manage it from the Extensions -> Plug-in Manager area.
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