Connect Sprint refinement and Sprint planning
There're two activities in the Scrum that are related to planning, the Sprint refinement and the Sprint plan. In the refinement, team will work together with the PO to have the user stories in the product backlog which are supposed to be developed by the team within next two to three Sprints elaborated, so that all those user stories are well understood before implementation. And those refined user stories will then be the inputs for the Sprint planning in which the team will typically pick up the high priority user stories and put them into the Sprint backlog based on the actual team velocity.
This process seems pretty simple and straightforward, but in the reality, it may not work as expected, and estimate the user story size at that high level may make the planning not that accurate. It could be debated that the estimation is relative, i.e. estimate the user story at hand by looking at a kind of "baseline" user story which has one story point. But, there might be many other questions for this approach, for example, what is the criteria of that "baseline" user story? who defined it? will that "baseline" be acceptable by everybody in the team? how to relatively estimate other user stories at the "user story level" while still keeping the estimation as accurate as possible?
In the practice, we usually using another activity which was not there in the standard Scrum framework, it is called as design workshop, which typically happen between the Sprint refinement and Sprint planning. This is essential that connect the refinement to planning and make the plan more accurate, because all the user stories will be further breakdown to lower level pieces by looking from the implementation perspective, such as models, views, controllers and entity objects.
By doing the design workshop, it is easily for the team to get aligned on what would be the high level design and what would be the physical objects to be implemented in the software, and based on this, it would be pretty easy for the team to do the estimation, by comparing those objects of the user story with those in the "baseline" user story which has already been defined by the team through design workshop, this is because it is always easy for people to compare two "concrete" things than to compare "abstract" things. Hence, it helps a lot to make a good Sprint planning.
In addition to connecting refinement and planning, design workshop also has many other benefits, such as help the team evolve a better software design, grow the software design skills of the whole team, and foster a better communication environment etc..
However, it also need to be balanced between the gain and the payment which is typically the time to be consumed, especially when everybody in the team is new to that activity. Design workshop also need to be defined and planned.