The latest Custom Platforms mod can be found in the can be found in the #pc-mods channel in the BSMG Discord or in Mod Assistant. 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.
Inside the game in the main menu you will see a PLATFORMS button at the top left of the Mods menu over by the release notes. Pressing it will open up the menu you can use to select which platform you want to use. Note that if you intend to use Progress Counter from Counters+, most platforms will block the center song timer.