Drag and drop an A2-formatted texture onto the Texture2D parameter on the RPGMaker_A2. This will slice the source sprite into Subtiles, as well as generate a RuleTile in that same folder. You should use a Tilemap on a Grid with half-sized cells, and paint your map with the included Modulo Brush. See the Examples folder for example usage & output.
This tool makes no assumptions about import settings for your tiles, but Assets/AutoTile/Examples/Editor/AssetPreProcessor.cs is used in the examples folder to adjust the import settings for the example sprites. This is a handy starting point for establishing your own default import settings, or it can be deleted to restore Unity's defaults.
I left the basic usage in the actual data ScriptableObject for my own reasons, but if you are feeling keen, its not a lot of work to adjust the functionality to a right click action.