Before you click activate.
BEFORE INSTALLING
Back up Check it is maintained Check reviews Know what it does
INSTALLING FROM THE DIRECTORY
Plugins, Add New, search, install, activate.
INSTALLING A PAID PLUGIN
Upload the zip file from the developer.
Only from the developer. Never from a third-party site.
AFTER ACTIVATING
Check your site loads Check key pages Check the admin still works
WHAT CAN GO WRONG IMMEDIATELY
A conflict producing a blank page A memory error Broken layout
IF THE SITE BREAKS
Rename the plugin's directory by FTP or File Manager.
That deactivates it and restores access.
CONFIGURING IT
Go through the settings deliberately.
Default settings are frequently not what you want.
WHAT TO TEST AFTER CONFIGURING
Whatever the plugin affects, and your contact form and checkout regardless.