Unfortunately it's not a reliable method, it involves opening up the phone and shorting the test point but if you must know here it is:
requirements: a heat gun, a pry tool, a small + screwdriver and something like a metalic tweezer and a sigma box or other alternatives.
oh you're also gonna need some E7000 glue to put it back together
You open up the phone, remove the battery shield, remove the battery connector, short
this point and plug in the usb right after.
After you just press unlock bootloader on your sigma key software
There MAY be better methods but this is all I can offer for now, it's really not worth spending $300 just for a bootloader alone unless you already have the tools.