The dd-wrt forums should help, you need an initial flash firmware that supports your EA6700 model and once that is flashed you can then update to any flavor build you wish
WARNING: Careful with 30/30/30 reset, apparently this is not recommended on EA series routers, also don NOT flash any build before 23213 2013-12-24 (from this build on your router is supported)
Yes you will void your warrantee, also you may brick your router... But I think its easy to go back to stock firmware
Ask in the forums for the steps you need for installing dd-wrt on your router