I didn't modify my own. Just flashed a BIOS that someone else had already modified.
I can try and find where the site is that I got the modified BIOS from if you like...
Or I could just send you the modified BIOS file? Up to you.
Should add that I used Trixx to Downclock/Undervolt my card.
Even with a modified BIOS Afterburner still wouldn't allow voltage adjustment.
Using Trixx without the modified BIOS allowed me to supposedly change voltage.. Always reverted back to original 1.163v once a game was fired up though. Hence the need for a modified BIOS.