Installation is very easy. Just follow the below steps.
Just navigate to the https://apps.shopify.com/mps-notify-me page and click on the 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 for your store URL (myshopify.com version), just insert your store URL where you want to install the notim.app and click on the Login button as shown in below snapshot.
Once you will click on the Login button you will see the next screen with the Install app button. Just press the Install app button as shown in the below snapshot and you will reach notim.app installation final step.
In the last step, you just need to choose your notim.app plan. notim.app has Paid as you grow plans. So just choose a plan that fits best in your store based on the number of products you are selling and the notim.app will start monitoring all products(and variants) of your store.