Log in to your Shopify admin. Go to 'Online Store' and select 'Themes.' Click 'Customize' for your current theme. Look for 'Sections' in the left-hand menu and click on it. Choose 'Add Section' or 'Add Content' and find the 'Countdown Timer' option. Set the timer's date and time, as well as its appearance. Save or publish the changes, and the countdown timer will be visible on your Shopify store.