How do I create custom webform element?
I am not sure if you are looking for a composite element or just a simple one, but I found a great example for a Composite element posted by the user lpalgarvio.

Using the example module makes it all simple, in this case you are extending the class WebformCompositeBase to implement your own composite element.

Another example would be to take a look at the elements which ship with Webform module and how each is implemented. According to the webform 5 (D8) module maintainer he is wanting to add such an example into the webforms module.

Update:
There are three examples of custom elements in the webform module itself, all quite clear and detailed:


Composite element - webform/modules/webform_example_composite
Element - webform/modules/webform_example_element
Handler - webform/modules/webform_example_remote_post
Config examples - webform/modules/webform_examples (pre-built forms)