The latest Custom Platforms plugin can be found in the Mod Manager. Run the Mod Manager, select the Custom Platforms plugin, and click Install.
Once you've installed it you will see a folder called
CustomPlatforms in your install folder, this is where you should place the
*.plat files you want to use. You can download more platforms on ModelSaber.
If you'd prefer a video demonstration, here is a tutorial by Helixx VR.
Inside the game in the main menu you will see a button called
PLATFORMS near the bottom right of the center panel. Clicking that will open up a menu where you can preview and select which platform you want to use. Note that if you intend to use the
Progress Counter mod, most platforms will block the center song timer.
Creating your own platform
Emma's Platform Guide is a great resource for learning how to make your own platforms.