I'm the happy owner of a R6250 and just installed the Kong Mod (from: https://www.myopenrouter.com/download/dd-wrt-kong-mod-netgear-r6250-2016...) on my router.
Firstly, I seeked help in forums (here: http://www.dd-wrt.com/phpBB2/viewtopic.php?t=51486)
And then I used the old installation guide for my R6300, because I'm a newb (here: https://www.dd-wrt.com/wiki/index.php/Netgear_R6300#Guides) and it's years in between flashing my fw. Though I made a 30-30-30 reset as the first reset, instead of the 'regular' factory reset.
Everything seems in order, though the Netgear logo doesn't light - which is okay, then I save some milli-amps when it's on ;-)
Now it's time to see if it behaves properly. Mad props for all people working on this and discussing this on these fora - then it's actually an option to use google and find proper instructions! ;-)