Go to your online store's theme customization. Select "Customize" for the theme where you want to display the countdown timer, and open the product template.
Click "Add section" or "Add block," then select the Apps tab. Add the app block named "Countdown Timer" and drag it to the position where you want it to appear.
Click the added block, then enable "Show demo in theme editor" to preview the timer in the theme editor regardless of selling period conditions. Customize the design and text, then save.
Open the target selling period rule and go to the "Before selling period" or "During selling period" section. Check "Show countdown timer (Only available on Online Store 2.0)," enter the title and description, then save.