Developing External Tool

Hello!

I am trying to develop my own external tool to perform certain calculations from a dataset, however I am lost to where to begin. Most of the documents and articles I have found online are instructions of writing the JSON file to connect the scripts to Power BI desktop. I would like to know how I would begin developing the script. Is there a template I could follow, or base code to develop on? I am not sure what functions/commands I would need to be able to retreive information from the datasets onto Powershell. 

Any tips, articles or documentation on how to write the script for the external tool would be of great help.

Thank you!