A little background first. I was actually preparing for PMI-ACP, and mid-way in my preparation and I felt Scrum Guide needs a thorough review.
After going through it a couple of times, and googling for my doubts withing it, I tried Scrum Open Assessment and got 95+ in first attempt. In that process, I also came to know PSM1 can be given without training, unlike CSM.
So I decided to give PSM1 a shot, and sat for the exam today. I passed with 94 percent.
Here are my 2 cents on how to prepare.