How do you make an srt file - is it possible to make it as a txt file and then just change the extension - what should such a file contain - especially to be properly synchronised with the film?

Yes, you can create an SRT file using a plain text editor and then save it with a .srt extension. The file should contain a sequence of subtitles, each with a number, the time codes indicating when the subtitle appears and disappears (in the format hours:minutes:seconds,milliseconds), and the text of the subtitle itself. For proper synchronization, ensure that the time codes accurately reflect the timing of the dialogue or narration in the film. Here's an example of the format:

<code>1
00:00:01,000 --> 00:00:03,000 Hello, world!
2 00:00:04,000 --> 00:00:06,000 Welcome to the movie.
</code>