Install a VST plugin onto your computer.
When selecting a destination folder to install to, select C:\\Program Files/VST Plugins. This is the default folder which FL Studio will search for VST plugins.
Go to Channels in the main menu.
Select Add One -> More
At the bottom of the window that pops up, click the Refresh button, and then choose Fast Scan option.
FL Studio will scan the VST folder, and add the new plugins it found and highlight them in red.
Put a check next to the plugins you would like to make available in the project.
Now you will be able to add a new channel powered by one of the VST plugins, by going to Channels -> Add One in the main menu.