Help with flashing DDWRT!

As the topic reads I need a few instructions on flashing DD-WRT on my AC66R(same as AC66U I am told, the official firmware downloads say that too, and the IC ID on the box is AC66U)

I already found the version needed to be installed on the router through the tool on dd-wrt.com but I am unable to find instructions on flashing.

Please help me out.

Thanks in advance