Open your PowerPoint presentation
Go to the slide where you want the timer
Click Insert
Click Text Box or Shape
Add the timer content manually, or insert a video/GIF timer if you have one
For a countdown timer, go to Insert
Click Get Add-ins
Search for a timer add-in
Install the add-in
Insert the timer onto the slide
Resize and position the timer as needed
Set the timer duration and options in the add-in settings
Save the presentation
Test the slide show to make sure the timer works
Use the Slide Show tab to present the timer during playback