Installation is very easy. Just follow the below steps.
Just navigate to the https://apps.shopify.com/mps-sales-notification page and click on Add app button as shown in the below snapshot.
Once you click on the 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 the Sales Notification app, and click on Login button as shown in below snapshot.
Once you will click on the Login button, you will see the next screen with the Install button. Just press the Install button as shown on the 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.