Installation is very easy. Just follow the below steps.
Just navigate to https://apps.shopify.com/mps-sales-notification page and click on Add app button as shown in below snapshot.
Once you click on Add app 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 Sales Notification 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 Sales notification app installation final step.
In the final step, all you need to do is choose your Sales Notification app plan. The Sales Notification app has both free and paid plans. So just choose a plan that best fits your store and the Sales Notification app will start to enhance the functionality of your store.