Architect RESTful API Microservices

Always remember that microservice you are developing will act as a façade whether you are solving simple or complex business problem.
Consider the scope of microservice and strict to be in the defined boundary.
Always be aligned with Restful Web API guidelines. Refer MSDN Restful Guideline, click here
Restful Web API can be developed for solving normal CRUD operations or for solving complex domain. In both cases, domain logic should be encapsulated with in microservice.
Create GET, PUT, and DELETE operations must be idempotent and not POST. An operation is idempotent if it can be called multiple times without having any side-effects after the first call.

Identify resources as Restful Web API should be resource oriented and should be stateless.
Identify endpoints with parameters, response, Http Status Code, and Http Verb.
If possible, mock the microservice API with mock data to share it with consumers otherwise design the JSON schema of responses with status code which can be shared with consumers.
Have an agreement on endpoints and JSON schema with consumers as it will save lots of rework. Document it properly and get the consensus from consumers before proceeding to development.
After above steps, start the actual implementation work using TDD approach.
And, if there is a need to interact with external API or service, consider using Façade or Proxy Pattern.