1. Track down developer API guide.
2. Identify authentication APIs.
3. See if they Postman collections.
4. Try the APIs in Postman
5. Use Postman code-gen-feature
6. Copy Postman code & place in a simple app
When you're comfortable with the basic Postman auto generated code:
7. Take the code and refactor it.
8. Place it into a class library.
9. Create custom objects to reflect data the API sends and receives.
10. You now have your own API !
More info: jamiemaguire.net