My students asked me a lot on how to prepare for Professional Scrum Master I (PSM I) Assessment, it is really easy if you are doing it right, two of my students just got 94% for the last class. 

1. Have a good understand of Scrum, especially the principles, the best is to practise, be Scrum Master or Product Owner or as part of the Development Team, real-life working experience in Scrum Team for 2-3 years is desired, If that is not possible, consider to take a Professional Scrum Master or Professional Scrum Foundations Course

2. Read Scrum Guide by Ken Schwaber and Jeff Sutherland, read it by heart and at least 4 or 5 times, I have personally read it at least 60 times and still found that there are a lot of things I miss during the read, it almost feels like it is new every time I read it.

3. Do Scrum Open till you got 100% multiple times 

The above Should be good enough for a pass, but if you are still not feeling confident, do the followings:

1. Read Nexus Guide as some questions is related to scaling scrum 

2. Do All open assessments including Nexus Open, Product Owner Open and Developer Open until you score 100% on all of them multiple time and some questions are related to specific areas like Scaling

3. Read books