Download the plugin installer or plugin files from the developer
Run the installer or extract the plugin files
Choose the correct plugin format for FL Studio if prompted
Install VST3 plugins to the default VST3 folder
Install VST2 plugins to a folder you can easily find
Open FL Studio
Go to Options
Open Manage plugins
Add your plugin folder to the Plugin search paths
Click Find plugins
Wait for the scan to finish
Enable the plugin if it appears in the list
Close the Plugin Manager
Open the Channel Rack or Mixer
Add the plugin from the plugin list
Rescan plugins if the plugin does not appear
Restart FL Studio if needed