This is the latest of several tutorials I’ve prepared on making the online timer tool visible in Zoom, updated to reflect the fact that OBS Studio software no longer requires an additional plugin to function as a virtual camera. The timer tool itself has also been updated and enhanced many times since the first version was released in 2017.
You could achieve the same effect using other webcam software, such as SparkoCam, ManyCam, or XSplit Broadcaster, but OBS has the virtue of being a free download from obsproject.com.
See also this related post on using the timer in the context of an online Toastmasters contest.
For more details on green screen setup, I recommend looking at this video tutorial: