Copy the mayan_custom_app folder to somewhere in your PYTHONPATH. When using Mayan with docker-compose, the easiest way would be to clone this git repo to /my/repo/path/mayan-custom-app and add the line
mayan:
...other sections...
volumes:
... other volumes...
- /my/repo/path/mayan-custom-app/mayan_custom_app:/var/lib/mayan/mayan_custom_app
to your docker-compose.yml.
Afterwards, go to System -> Setup -> Settings -> Common in the Mayan web interface and edit the COMMON_EXTRA_APPS setting to the value
- mayan_custom_app
Finally, restart Mayan.