This is how I prepared for the Elastic Certified Engineer exam.
Watch the video
This video gives a good overview and even shows a couple example problems. It's an hour long so get comfortable.
Read the FAQ
This FAQ has a lot of logistical information regarding the exam.
Register for the classes
These aren't cheap but they are worth it, even if you already have a lot of experience with Elasticsearch. Get your employer to pay for it.
If you plan to register for Engineer 2 after you complete Engineer 1, you may find that your ideal Engineer 2 offering has filled up and you'll be stuck taking it at an inconvenient time.
Register for both classes up front.
Copy and paste the exam objectives outline into a markdown file.
Get familiar with what topics are listed here.
Take notes directly into your objectives outline.
The classes don't follow the order of the outline but luckily you familiarized yourself with the outline beforehand so it's easy to jump around.
Finalize your outline
Go through your outline notes and use the class slides or Elastic docs to fill any gaps.
Be sure to switch the dropdown in the docs to the Elasticsearch version that the exam covers.
Install Elasticsearch locally, create some indices and work through some of the labs from class. Use the outline as a guide for choosing what to practice.
Day before the exam
Watch the video again. Seriously.
It's like repeat viewings of a good movie. You'll pick up on stuff you missed the first time through.
During the exam: don't panic
The video reminds you not to panic during the exam. This is important.
The test environment will get buggy. Your connection will get laggy. You will get stuck on a question.
You need to stay composed and keep moving forward through the exam problems.
After the exam: exhale
Sit back, relax and wait for that congrats email :)