If it was difficult, it might be the case that you perhaps are not able to look at yourself objectively, or have not given it a thought. If it was easy, that means you know yourself and can look at yourself objectively.
In today's world, you cannot afford to not know yourself, your strengths and weaknesses.
It is going to be a big help in the corporate world where you may be asked to evaluate your performance yourself, as employee evaluation is an important aspect in the professional arena. You have to balance between correctly portraying what you have been really good at as well as areas where you might have lagged behind.
Moreover, it is a stock taking of sorts of an employee's quality and quantity of work by himself, goals and performance enhancement, and future plans for career growth.
How to Write an Effective and Powerful Self-evaluation?
Reserve some part of your day for this work. Ensure that no matter what happens, you work on evaluating yourself, and try and finish the whole process in a week.
If you have worked really hard (please be honest here) for the company, then have no qualms about praising yourself. At the same time, be humble. Usage of strong action verbs like produced, received, managed, created, and so on will have a positive impact.
If it was the latter, describe about your effective communication skills, team adaptability, and focus on achievement of objectives.
Have a good detailing of your overall performance, including your strengths, future aims, growth and development.
To conclude, state what you can do and have done towards the growth of the company and create new and more business opportunities.
To perform self-evaluation for a positive performance review, all you need is to be honest and objective. At the same time, don't hesitate on giving yourself the due credit where it needs to be!
After all, at the end of the day, in such a review, a little bit of self obsession and promotion would not be such a bad idea! Good Luck!