If you wish to participate in the examination of an application for development consent, you first need to register with the Planning Inspectorate by making a Relevant Representation about the application.
Short film: How to have your say on Nationally Significant Infrastructure Projects
Advice note 8.2 provides information on how to register and become an Interested Party. Further, more detailed, advice about participating in the process can be found in advice notes 8.1 to 8.5.