Installation is very easy. Just follow the below steps.
Just navigate https://apps.shopify.com/mps-promote-me page and click on the Add app button 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 Promote 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 on the below snapshot and you will reach to Promote Me app installation final step.
In the last step, you just need to choose your Promote Me app plan. Promote Me has Paid as You Grow plans. So just choose a plan that fits best in your store and Promote Me will start displaying its amazing widgets in your store.