The thing that you have to keep in mind is that you have to do a stable amount of research conducted. If you are on the spectrum of avoiding conflicts, to avoiding uncomfortable situations then you may find it hard to be a Mystery Shopper. Because being a Mystery Shopper is all about testing how the staff of the business is trained to handle your complaints.
The second thing would be to properly train yourself or join certain firms who train their employees for this task. Most times, companies will prefer those individuals who are properly trained and equipped for the job.
Then finally, you can register by putting in your credential information on the registration page and become periled on the mystery shopping adventure!