Define your goals. Understand your target audience. Refine your message. Write text content. Gather and/or create media resources (images, graphics, video). This critical stage requires communication and teamwork from both parties.
A free, cloud-based, project management board (Trello) will contain all the information any developer would need in order to move into the prototype stage.
The sample is set for public view. Your project would use a private, invite-only board.