One way to get started developing a miRMaid plugin:
* After setting up miRMaid locally, create a new directory for the
plugin in vendor/ (i.e. 'vendor/mirmaid-targetscan'). This directory
holds all code for the plugin, so it can be put under version control
on i.e. github.
* start developing the plugin in this directory. I guess one could
start from scratch by iteratively adding unit-tests, features and
implementing code, or by taking an existing plugin and gradually
modifying it towards the goal.