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 :
https://forum.xda-developers.com/z3-compact/general/recovery-root-mm-575-lb-t3418714
...without any success.