Connecting 12V WS2811 RGB LEDs to Arduino Uno & 5V Power Source (Breadboard needed?)
This image, from here, shows which pins are what:



Note the flat key next to the 12 V and GND pins. In addition:


the GND wire has a black broken line printed along it;
the data line is in the middle. 


If you trust the image then you should be able to wire them up correctly.