It's pretty easy to install a script plugin & following the steps below will help.
Note a unique license is generated on purchase which can be used to activate the plugin in the script. This is essential to use the plugin.
All you To do that you need to navigate login as admin & navigate to "Admin - Plugins - General Plugin". From there you would upload the downloaded plugin.zip. Once done you need to add your purchase license to activate the plugin.
For the customization plugin. There's a readme in the customization plugin for installation guide.