To develop a custom workflow step in Adobe Experience Manager (AEM), a developer should follow two key steps. First, they need to implement a Java class that adheres to the workflow process interface by implementing the interface com.adobe.granite.workflow.exec.WorkflowProcess. This ensures the custom process can be executed within AEM's workflow engine. Second, the developer must create a Workflow component node with the Super Resource Type 'cq/workflow/components/model/process' under the folder /apps//components. This sets up the necessary configuration for the custom workflow step within AEM’s JCR repository, making it recognizable and usable within the workflow models.