XS++ 3.1 Guide-Flashing,Customizing,Recovering - Top Sony Forums
4) Customizing your phone
Compulsory if flashing FS
Step 1.
Take battery out of phone and put back in (do not turn phone on). Open XS++ and click Connect. Hold down the "C" button on the phones keypad and connect the USB cable.
Cann't connect and see Error: Cannot open phone communication port?
Read here: USB Flash Driver Installation - Guide - Top Sony Forums
And see screenshot of 1 page.