Refine the backlog · Pecos Mutual Product Operations

When to use

Hold a refinement session in the middle of every sprint, about day 5. Keep refining between sessions as questions come up.

Product Owner: leads the content. You bring the ordered backlog and answer questions about value and acceptance criteria.
Scrum Master: facilitates and keeps the session to its timebox.
Developers: ask questions, split items and size them.
Product Manager (optional): joins when items need a decision on scope or value.

Check the goal. Restate the next milestone and its "done when", so everyone refines towards the same target.
Walk the top items in order. For each one, the Product Owner explains the value and the acceptance criteria.
Ask and answer. Developers ask what they need to know to build and test the item. Note any question you can't answer in the room, with an owner and a due date.
Split large items. If an item won't fit in one sprint, split it into smaller items that each deliver value on their own. Split by user journey or business rule, not by technical layer.
Size the items. Use the team's usual method, such as story points. Re-size anything that has changed since it was last sized.
Check against the Definition of Ready. Mark the items that meet it. Note what's missing for those that don't.
Check dependencies. If an item needs work from another team, confirm the date with that team before you mark the item ready.
Stop at two sprints. Once roughly two sprints' worth of items are ready, stop. Refining further ahead wastes effort on items that may change.

Designing in the session. If a conversation needs a whiteboard and an hour, take it offline with two or three people.
The Product Owner refining alone. Items the team hasn't seen aren't ready, however well written they are.
Splitting by layer. "Build the API" and "Build the screen" each deliver nothing on their own. Split so each item can be demonstrated.