Define the usability problem.
Assimilate all of the research and information gathered, and synthesise into the core problems.
A clear way achieving a common understanding with these problems is user stories: define who the problem affects, what happens if the problem was not solved, where the problem is taking place, when the problem needs to be fixed, and why it is important for the problem to be fixed.
For example a short problem statement: "As a [persona], I want to [do/have something] so that [reason]."
Use these problem statements to then define personas, and pick scenarios to focus the design around.