There are three ways I know:

1) Using only Phone XS or XS++ - This means this programs to do all of the three: upload MAIN and FS and do a "brute force" to set customization.

This one doesn't work for my K310. Neither XS++ nor Phone XS.
2) Using only FAR Manager with SEFP (Sony Ericsson Flash Plugin) - you need to upload special MAIN (which is called 'breaking') which allows FAR to intrude your phone without "brute force".

This one works only partially for me. I cannot access FLASH, only FS.

3) Using WHAT-IS-POSSIBLE :D
I do this this way:
- first upload FS with Phone XS v2
- make a customization with FAR (First you need 'breaking' MAIN for this of course)
- upload MAIN with either XS++ or Phone XS (I used XS++)
- enjoy your new firmware and thank all TopSony users :D

There are GUIDES here in TopSony which describes what you need to do if you wanna use Far Manager or Phone XS/XS++.
See Tutorials & Guides