cordova plugin add <plugin-name>
Example: cordova plugin add cordova-plugin-camera
The plugin should be installed in the root directory of the project.
Before adding the plugin replace the platform specific www folder content with the outer www folder in the root directory. This is because on adding a plugin the contents of the outer www folder are replaced with the platform specific www folder.
When you add a new platform and if you have any installed plugins on your project is not necessary to install the existing plugins. Cordova automatically will add the installed plugins for the new platform.