Self-assessment can be as formal or informal as you wish to make it
Make sure to take time away from a normal work day to reflect and do your personal assessment.
Ways to do a personal assessment: - Work with a coach to talk through areas you want to work on - Doing your reflection with colleagues for a 360 review - Formal assessments that can be found online or in books include: Myer-Briggs, or Birkman Attitude assessment
Make sure you are open to really hear and understand the feedback you receive. People take time and effort to give you information to improve yourself: that is why 'feedback is a gift'.
Watch this great storyteller to help you understand the value of truly listening to the feedback you get: