Using MKVToolNix GUI v56.1.0 to create Chapter Mark Files and Split at Chapter Marks
http://priede.bf.lu.lv/ftp/pub/MultiVide/video/mkvtoolnix/apraksti/mkvmerge-gui.html#chapterformats
Here's another source:
Splitting multi-episode files with MKVtoolnix GUI - General / Tips, Tricks & How-Tos - Plex Forum
https://forums.plex.tv/t/howto-joining-multi-part-movies-files-with-mkvtoolnix-gui/113211
Well fast forward a few years and I discovered a way easier free program for naming chapters called chapterEditor, and you don't have to save them to a file first, nor is there any processing. You simply type in the chapter and click Save. It instantly writes the chapter names with no wait time.
https://forum.doom9.org/showthread.php?t=169984
