Once you're done with the steps described previously, it's time to put all the requirements you've gathered and refine them together in a single document. It doesn't matter what form the document will take and how you will manage it. What matters is that you have a document that puts all the stakeholders on the same page with regard to what is required from the product and what value each requirement brings.
Requirements are produced and consumed by all stakeholders, and a broad set of them will need to read your document. This means that you should write it so that it brings value for people of various technical skills from customers, salespeople, and marketers, through designers and project managers, to software architects, developers, and testers.
Sometimes it makes sense to prepare two versions of the document, one for the people closest to the business side of the project, and another, a more technical one, for the development team. However...