1. Make sure you've "scoured" the official Certification page - drilling down on detail for the particular exam you intend to take.
2. Get the detailed "JTA" (job task analysis) for the exam. You can get them at CTS (CTS-I, CTS-D) JTA | AVIXA Xchange
3. Consider purchasing the McGraw-Hill/AVIXA Exam Guide book/ebook. As of July '23, CTS is in 3rd edition, CTS-I is in 2nd edition and CTS-D is in 2nd edition. These aren't just great study references - they are trusty "on the job" resources for the AV pro. McGraw-Hill/AVIXA exam guides are at most major booksellers including Amazon.com, McGraw Hill and Barnes and Noble
4. Sit down and go through the detailed JTA (see step 2 above); slow and purposely. For each task presented ask yourself how strongly (or not) each resonates with you. If you're like ".. oh yeah, I'm very comfortable on how to do that! .." then move to the next. But, be honest here, if the response is more like ".. pretty sure I've seen that done .." (or similar), then highlight that for further study.
5. Take your marked up detailed JTA from step 4 and start brushing up on all the items you highlighted. The Exam Guides have Chapter references back to those very same JTA tasks, so it's relatively easy to cross reference what section(s) you'll want to (re)read. There is also detailed TOC (table of contents) as well as Glossary to help. In addition, you can see if there are any other courses (or content) that will help; AVIXA content team continues to make more "micro" course content available. Just take key words from the JTA task descriptions you highlighted and plug them into the search tool of the Training Catalog | AVIXA