Installation is very easy. Just follow the below steps.
Just navigate to https://apps.shopify.com/mps-buy-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 Buy Me 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 button. Just press the Install button as shown in the below snapshot and you will reach to Buy Me app installation final step.
In the final step, all you need to do is choose your Buy Me app plan. The Buy Me app has both free and paid plans. So just choose a plan that best fits your store and the Buy Me app will start to enhance the functionality of your store.