First, model the happy path to the desired end result before collecting problems and exceptions, prioritizing them, and introducing them incrementally. Secondly, focus on one selected issue at a time, and choose the right techniques for modeling beyond the happy path.
To start a new process instance or to route a message to an already running instance, you have to choose the appropriate technology option to do so, like using the existing API or using customized possibilities including SOAP, AMQP, or Kafka. Leverage the possibilities of the universe of your runtime (like Java or Node.js) and the frameworks of your choice to support the technologies or protocols you need.