Open PowerPoint and go to the slide where you want the timer
Click the Insert tab
Select Text Box, then draw a text box on the slide
Type the timer value you want to display
Go to the Animations tab
Choose an animation for the text box
Open the Animation Pane
Set the animation to start Automatically or On Click as needed
Adjust the duration in the Timing section
Use multiple text boxes or duplicate slides to create a countdown sequence
Set slide transitions to advance automatically after a chosen time
Go to the Transitions tab
Uncheck On Mouse Click
Check After and set the time for each slide
Use the Slide Show tab to test the timer
Save the presentation