Installing TWRP on locked Sony Z3 Compact D5803

I was googling that for ages and couldn’t find any decent solution.

I have locked Sony Z3 Compact [D5803] and I need to install TWRP and backup the current OS.

I CAN use flashtool to install custom files that had been downloaded from xperiFirm but it seems like impossible mission to root it afterwards 🙂

I was also trying this article here :

…without any success.