Is there a guide or wiki on how to create a plugin compatible with Cura LE 3.6
Ultimaker has some documentation. See:
https://ultimaker.com/en/resources/21966-cura-plugins
https://github.com/Ultimaker/Cura/wiki/Plugin-Directory
Many of the plugins are provided in source (.py) form so you can usually find a good starting point and modify it from there.
Post processing plugins are found at C:\Program Files (x86)\cura-lulzbot 3.6\plugins\PostProcessingPlugin\scripts.