Any guidance on the steps for how to attach the tools create connections for a custom element into a standard bpmn diagram /modeler would be much appreciated.
No connections are allowed for custom elements, unless you are adding a rule for it. You can take a look at bpmn-js where all connection rules for BPMN elements are implemented: https://github.com/bpmn-io/bpmn-js/blob/master/lib/features/rules/BpmnRules.js. You can add connection rules for custom elements in the same way by extending customRules.
Thanks for that! I will give it a go.