how do I power and control 300 WS2812B RGB LEDs?

Other Hardware LEDs and Multiplexing
1

I have an Arduino Uno (r3). What's the easiest way to power and control a 300 RGB LED WS2812B strip?

2

Control - use the Adafruit library for neopixel.

Power - that will need up to 3 x 20mA x 300 LED = 18A.
You may need several smaller supplies connected at intermediate points along the strip.