SS to FG

Let me first say, I am a numpty when it comes to mechanics ... hence what is to follow!

I have a standard 2012 Charge Plug with the flip-flop rear wheel which is currently a SS. I want to convert this to FG. Exactly what do I need to do this? Ideally I'll stick to 16 tooth on the back so I assume chain length remains the same?

• So do I just need a rear cog? If so, any recommendations?
  
• Do I need a locking ring, or would I not need this on the Plug?
  
• What tools do I need (apart from myself)?
  
  

Cheers in advance :)