Create an empty atom, click on the empty atom then click the Plugins tab, Add Plugin, Select File, click the CS file.
Click Open custom UI to change the settings
Here's a quick guide
https://docs.google.com/document/d/1lyg5DHPZgrHBgDmOC3UwWJ5fuAb7nqhDCiqf2laMoZM/edit?usp=sharing