1. Enable the app.
2. Choose the maximum inventory for which you want to display the app. You need to decide on an inventory threshold to use - all products / variants with an inventory smaller than that that amount will show the app.
The Stock Urgency cannot be displayed on products with a higher stock without displaying the real one. Shopify doesn't allow fake information on its stores - https://www.shopify.com/legal/aup - so back in the summer of 2017 it banned all apps that provided fake stock information or fake recent sales notifications.
If you want to use the app on most of your products, you can choose to put a threshold of ~1000 or more and add a general message like "Hurry, only a few in stock" (so not to show the real value of inventory), but you risk infringing on Shopify TOS and it will look artificial if all product pages will carry the same message.
3. Customize the text that will be displayed with the app.
4. Choose if you want to show the urgency box when a product variant is out of stock. Customize the text for it.
5. Customize the colors of the app.
6. Do not forget to save the changes.