Once you have a list of topics for which you need preparation, you may face two challenges.
Find and study documentation and knowledge resources for the topics, and
Find enough practical case studies covering the topics.
Finding documentation and online resources is easier. You may register for Acquia Academy Courses and take a course in the relevant domain to refresh your knowledge.
A list of courses on Acquia Academy can be found at: https://training.acquia.com/free-training. Depending on your areas of weakness, you may wish to take one or more of the following courses:
Introduction to Drupal 8 (Introductory Level)
Building a Basic Site Using Drupal 8 (Introductory Level)
Extending and Managing a Drupal 8 Site (Introductory Level)
Building Websites with Drupal 8 (Intermediate Level)
(Note: as Drupal 8 and 9 are very similar in architecture and concepts, you may start with the latest of the Drupal 8 or Drupal 9 courses from Acquia Academy)
You can access these courses for free, and review them as many times as you like. Many Acquia Academy courses are available, and if you are in an Acquia enablement or training program, you have been assigned the courses required to obtain an initial certification.
Courses follow a model of 100-level for introductory material, 200-level for application of the concepts, 300-level for advanced concepts, and 400-level for solution architecture. Those with more exposure to Drupal 8 concepts, or with demonstrated site building experience with Drupal 7, may skip the D8 100-level courses.
We also include a list of online resources and documentation for each learning domain in the We also list of online resources and documentation for each learning domain in the Study Guide Section of this document.
The second challenge of finding sufficient practical case studies covering the topics is likely to be more daunting. Here are a few ideas and suggestions for finding case studies for your preparation:
Review several (3 to 5) Drupal 8 projects that you or your team has worked on in the past year. Review the content types, field settings, display settings, vocabularies, blocks, menus, and views used on the sites.
What are the Content Types for the site?
What are the fields and field settings for each Content Type?
What are the Displays configured for each Content Type? Are there any Custom displays? If so, how and where are they used?
What Vocabularies are defined? Are they controlled or uncontrolled? How is the Taxonomy system used?
How many Custom blocks are used on the site? Are there any Custom Block Types defined? Are there any custom fields in the blocks? How are they used on the site?
How many Menus are defined? Are there any Custom Menus? How are the Menu items managed? Where are the Menus used on the site?
List all the Views used on the site. For each View, check the Displays, Format, Fields, Sort Criteria, Filter Criteria, Contextual Filters and Relationships. Check how views are used on the site.
List all the Custom Modules enabled on the site. Check the functionality added by each module.
Go to the Configuration Settings page of the site. Check the settings made under all the available options. Check the settings made under People, Content Authoring, Development, Search and Metadata, RSS Publishing, System, Media, Regional and Language Settings.
Go to Extend. Enable modules under Multilingual section. Check the different configuration options newly available under Language settings and Content type configurations. Play around with the settings to build a bi-lingual or a multilingual website.
Why are the configurations the way they are?
What can be changed or improved?
What are the issues or support requests raised by the client?
What changes were made to initial configuration and why?
Learn more about the Acquia Certification Program at https://acquia.com/certification