You need to have a method that can accept a HTTP POST request. Typically this is done using a web framework. This is what a very simple implementation of the callback looks like in Django having csrf turned off, which is not recommended, but is ok for illustrating how callbacks can be setup.
Use localhost tunneling software to generate a URL to your callback handler. This article has some suggestions on this process.
If you used this handler and tested on HelloSign.com, you'd get the Success! message because the callback page sent the string
response = HttpResponse("Hello API Event Received")
back to the tester
For more information, please check out our Events and Callbacks Walkthrough
It is important to note the section on Event Hash Verification because that is important for security.
Another helpful link is the Python Django Demo app, which shows a callback example.