I didn't build my own BIOS per se.
It's extracting the existing BIOS and re-writing it to have higher TDPs and re-mapping the voltage, boost and power tables. However all this comes with the downside of generating immense amounts of heat and power consumption.
Also not all cards are able to do overvolting. Not only you need a unlocked BIOS, you also need a programmable voltage controller on your card. In all over volting tools, AB or otherwise, you need the controller to listen to what you command. If not, you can only go as high as what the gfx card physically permits which is probably +50 to +100 on core based on my experience with locked cards like the GTX760.
If you're are a crazy overclocker like Yureak and myself, get a EVGA Classified card. They overclock like the mother of all overclocking cards.