The file browser is a great way to quickly find both samples and presets to use in your productions. But how do you add your own folders? It's incredibly easy.
Navigate to the toolbar and click Options -> File settings. Look for Browser extra search folders and click one of the empty folders.
When clicking on one of these, you will be able to navigate to your own custom folders where you keep your samples and presets. When done, just click Ok.
Now go into your browser inside FL Studio and click the little arrow (marked in green) to reread the browser structure.
Now you'll see your own custom folder inside the browser. This can be done over and over for all of your own content.