There are multiple techniques can be adopted to support the PO during backlog refinement session to keep the PBI well-refined. However, irrespective of any techniques, the main goal is prioritisation should be in such way that would deliver maximum business value by end of upcoming sprint. Here's few on how to achieve that.

MoSCoW technique : The stories can be prioritised based on importance of functionalities (Must have, Should have, Could have, Would have)

Regarding the defining the stories, it should meet INVEST criteria (Independent, Negotiable, Valuable, Estimable, Short, Testable)

Regarding the sizing, the bigger stories should be splitting into small stories based on Workflow, user role, business rules etc. We can use T-shirt sizing for epic level stories to further break it down.

I work closely with Product Owners to keep the Product Backlog healthy. For this, I support POs through regular Backlog Refinement sessions, usually once a week. During these meetings, I encourage Scrum Teams to engage and review upcoming backlog items actively, discuss priorities, and ensure everything is aligned with the latest business goals.

In this meeting, we collaborate to split big items into smaller User Stories using SPIDR technique and ensuring stories follow INVEST criteria (Independent, Negotiable, Valuable, Estimable, Small, Testable). This helps us create clear and actionable user stories. For sizing, we use planning poker during our refinement sessions. This involved the team in estimating effort, which also improved their understanding and buy-in.

In addition to this, I work with the Product Owner to prioritize Product Backlog effectively. We use techniques like MoSCoW, WSJF, RICE and the Kano model, depending on the context and maturity of the business unit.

Finally, we maintain open lines of communication with stakeholders and the development team. Regular feedback loops ensure that the Product Backlog reflects the most current needs and insights.