Two theme editor pieces, and only one is required
The app embed is a switch under App embeds in the theme editor, listed as Countdown Timer. It loads your published timers onto the storefront, so no timer shows anywhere until it is on. It is the only thing you have to turn on.
The app block, listed as Countdown timer in the Add block menu, is optional. You only need it for a timer set to Custom position, or when you want a timer somewhere other than its automatic spot. Automatic spots are fixed per timer type: below the Add to cart button for a product timer, across the top or bottom of the page for a bar, at the top of the page content for a landing timer, and at the top of the cart page and cart drawer for a cart timer.
Turning on the app embed
The app Home shows whether the embed is on in your live theme, with a button that opens the theme editor at App embeds with ours selected. Turn it on and save the theme. There is nothing to configure on it for most stores. Its one setting, a cart drawer selector, is only for a theme whose cart drawer the cart timer does not find on its own.
Placing a timer with the app block
In the timer Placement tab, choose Custom position, then save or publish. The tab then shows the Timer ID and an Add app block button. The button opens the theme editor on the right page type, a product page for a product timer, a collection page for a landing timer, the home page for a bar, and the cart for a cart timer, with the block being added. Paste the Timer ID into the block, move the block where you want it, and save.
A block can also hold a timer that keeps an automatic placement. It then shows only on pages that match the placement, and the timer is skipped at its automatic spot on that page, so it never shows twice. In the theme editor an empty block shows a short hint explaining why. Only you see it.