If you're powering the sword with a AAAx3 battery pack, flip the switch from OFF to ON.
If you're using a LiPo battery, simply plug it in the the JST connector.
When you turn on the sword it will take a couple seconds to calibrate itself. Don't touch it while it's calibrating!
If you find the the NeoPixel strip isn't responding as you'd expect, you can press button A on the Circuit Playground Express to re-run the calibration. Just be sure to put it down and don't touch it for a couple seconds while it's calibrating!
If you enjoy MakeCode and want to continue exploring you can check out lots more MakeCode projects on the Adafruit Learn System.