The canonical link for “Guide: Installing SailfishX on Xperias” is this one.
I still maintain it and update it occasionally. Please report issues with this guide at its issue tracker there.
The version at Together.Jolla.com (TJC) is outdated, because the TJC forum was switched to read-only in the year 2020.
Hence you really should use the section 3.3 Increasing the “root” LVM volume size at Gitlab, not the stale one at TJC or the ones from the year 2019 (each only covering a specific, unencrypted device) by Speefak, PCFE or seiichiro0185.
As always: Please do read thoroughly before applying such critical steps.
Mistyping, executing steps in the wrong order etc. may arise the need to perform a “factory reset”, to re-flash or even brick your device.