The Product Owner establishes a first list of requirements (a requirement becomes a Product Backlog Item) he wish to be implemented for the end of the release.

To complere this list and obtain a sufficient backlog, it is better to organize a workshop including the whole team plus stakeholders. During this workshop, the Product Owner introduces his backlog draft and each one may suggest additional items.

It is frequent a backlog item be identified as a User story, technique coming from EXtreme Programming (XP) et fitting very well to the Scrum pattern.

The Product Owner prioritizes backlog items by decreasing priorities. To him, A more important than B means he wants A to be implemented before B across the series of partial products.

wjem a backlog contains a lot of items, the prioritization of each of them is fastidious and subtil. That's why it is useful to define Themes, associate one to each item and prioritize themes before getting to item prioritization. This can be done during a meeting with team members and stakeholders.