Someone recently just smash it n broke completely. I google some replacements but some reviews said they seemed kinda off. Any recommendations where I can get an oem one ?

 

Is your 2012 a Journey?
2012 Infiniti G37 Journey Outside Mirrors

 

No, it's 2011 sport

 

OK then, in that case:
2011 Infiniti G37 Sport Outside Mirrors

 

How hard is it to change the mirror - my drivers side got broken off - I already have the assembly and need to install it... is there a step by step? I have a 2011 G37x Sedan. Thank you!

 

http://www.nicoclub.com/manuals/G37/Sedan/2011/MIR.pdf
page 18...
And page 12 of the INT manual for help removing the door panel:
http://www.nicoclub.com/manuals/G37/Sedan/2011/INT.pdf
I bought some spare "clips" from the dealership to have on when I do this kind of thing because I inevitably break a few... But this operation looks doable. Good luck.

 

Thank you for the excellent information and links to this information. You are awesome. I will be doing this over the long Christmas weekend.