You have to know some information about your phone.
And the Information you can reed with free programms like XS++ or SEtool Lite. You have to know the Erom CID(Not OTP). And when you know the CID you have to download the firmware wit the same CID, the same model(k790i) and of couse not branded firmware (e.g Main-Generic and FS with the languge pack you want e.g Europe2,Europe3,Europe4,America...)

And last you need also unbranded cutompack.
All these things you can download on topsony.com

Unbranding is very easy ;-)

Please ask when you have more questions