The latest Custom Saber plugin can be found in the Installer. Run the Mod Manager, select the Custom Saber plugin, and click Install.
Once you've installed it you will see a folder called CustomSabers in your install folder, this is where you should place the *.saber files you want to use. You can download more sabers on ModelSaber. There is a limit to the number of sabers that varies by machine, but the average number of sabers supported seems to be around 10.
Inside the game in the main menu you will see a button called SABER MENU at the bottom right of the right panel. Clicking that will open up the menu you can use to select which sabers you want to use. Note that once selected the sabers will show up only while a song is playing, and not while in a menu.
angeloid0103's Saber Guide is an excellent resource for learning how to make your own sabers. Note: at one part it tells you to create a new project and drag the template project files into that project—that is incorrect; you need to open the template project with unity instead, not create a new one.