I know how to do that with Gnome Subtitles:
opening the subtitle and the video
pausing video when the proper subtitle line is selected
adding a sync point at the beginning and at the end
This fixes the subtitle in 70% of cases. Adding 2-3 more intermediary points brings that to 95%.
In KDE I would prefer to use Subtitle Composer, which is Qt/KDE tool. It has many other advantages. It seems lighter, richer, with better video rendering (scrolling video is awful in Gnome Subtitles). It is cross-platform, and in Linux is also available as Appimage.
All I could find found about synchronizing with this tool is:
Quick and easy subtitle sync:
Dragging several anchors/graftpoints and stretching timeline Time shifting and scaling, lines duration re-calculation, framerate conversion, etc. Joining and splitting of subtitle files
But what exactly are the steps to sync a subtitle with this tool?
What steps are the equivalent of those taken in Gnome Subtitles?
Edit:
Sometimes this simple trick works: select the subtitle line that you need to synchronize with a line in the movie, stop the movie at that moment, and press Shift-A (or Times - Shift to video position) and save.
But in most cases it's not enough.