Download and extract to your SD card and reboot. It should be active after reboot. Additionally, download Ultrahand which is an up-to-date replacement and enhancement of the original Tesla Menu for managing overlays and sys-modules.
If you...
Just took a quick look at it. No idea how to use it. I've already got my Switch on the newest firmware and atmosphere, just using the fusee.bin that came with the release. How do I use it in that case?