Establishing convincing arguments for your design solution
By conducting complementary and regular evaluations at different stages of your design process using the appropriate evaluation methods (e.g. summative, formative, expert-based, observational, ...)
Evaluations should result in insights regarding possible problems and their causes in order to support refinement of your design specification
Look at user experience in its full breadth: Effectiveness, efficiency, satisfaction, learnability, mood, connectedness, ...
