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. (For an alternative that does not require additional software, see the Blue Sky Apps timer.)
See also this related post on using the timer in the context of an online Toastmasters contest.
If your club uses the WordPress for Toastmasters club website and agenda system, the timer tool is available under the agenda menu for regular meetings. The timer is available independently at https://toastmost.org/rsvpmaker/timer/?timer=1 — although you lose out on the benefits of integration with the agenda with that standalone version.
Update: Using the popup timer, rather than the main screen.
For more details on green screen setup, I recommend looking at this video tutorial: