When your definition is ready you may test it in the Analyzer/Data tab as follows. You may also generate a test sample with the Analyzer.
After testing a sample you can see the value of each element in the message structure and the errors (if any) to correct.
The final step is to generate the XSD. To do that just navigate in the main menu to File -> Export and select "Oracle 2.0".
You can generate the xsd with default configuration or check the box "Show Advanced Option", in which you may, for example, set the namespace and the structure of the field names. In the example below I'm specifying the field names as NodeID_NodeName.
In the following window of the wizard it's recommended to check the option "Suppress Enumeration in XSD" if you have code lists, otherwise they would be included in the XSD, since the ECS is used for validating and translating this information is not needed in the XSD and reduces it's size considerably.
After finishing your document definition and generating the schema you are ready to create your B2B document definition. However have in mind that for Positional Flat File documents, as in this example, you also need to generate the Parser ECS.
For information on how to generate the parser schema check out the post Oracle B2B 12c Positional Flat File and Charset Configuration