Its a very boring task but all you have to do is.
Step 1: Set all your die volt offset to -0.1245volt if you are running default clock 475Mhz.
Step 2: Restart the miner and wait around 20min for the controller to start all the dies, if you see a die not running after 20 min lower the offset to -0.1172volt and restart the miner again and so and so.
Like I said its slow proses and also depend if you were lucky with quality of the silicon used to make your cube.
I can normally under-volt from -0.0586 volt all the way up to -0.1172Volt running default MHz. And I don't think I have especially lucky cubes.