I’m having trouble adding the subtitles to the AVCHDs in the intent of burning to a DVD. To begin with, operating on an old MacBook doesn’t make this any easier. Might just be that my computer skills aren’t sufficient enough…

I have the AVCHD files for the movies in the .ISO form, which means the BDMV and CERTIFICATE folders.
TsMuxerGUI won’t read the m2ts files. I’m getting the “Can’t detect stream type” error when trying to add the file to the input. Which “video and audio file” does it mean in the Project Threepio README?

Am I doing everything wrong? Should I have the films in a completely different file type in the first place? Should I be converting or demuxing the files, if that’s even a thing? - can’t be done in TsMuxerGui since it “Can’t detect stream type”.

If I’m completely on the wrong track here, don’t bother trying to help me! I can happily watch the films without subtitles, they would be just a nice bonus.