Installation is very easy. Just follow below steps.
Just navigate to https://apps.shopify.com/mps-notify-me page and click on GET button at top right corner as shown in below snapshot.
Once you click on GET button if you are not already logged in then Shopify will ask your store url (myshopify.com version), just insert your store URL where you want to install Notify Me app and click on Login button as shown in below snapshot.
Once you will click on Login button you will see next screen with Install button. Just press Install button as shown on below snapshot and you will reach to Notify Me app installation final step.
In last step you just need to choose your Notify Me app plan. Notify Me has Pay as You Grow plans. So just choose plan that fit best on your store based on number of products you are selling and Notify Me will start monitoring all products(and variants) of your store.