Here are the detailed steps for subscribing to a Google Maps API key:
Create a Google Cloud account if you don't already have one by visiting cloud.google.com.
Connect to the Google Cloud console using your login details. Please note: it is imperative that you provide a means of payment, otherwise your account will never be active.
Create a new project by clicking on the 'Create a project' button in the top left-hand corner of the console.
Give your project a name and select a project number if necessary. Click on the 'Create' button to create the project.
Access the API management console by clicking on the 'Keys and identifiers' menu in the left-hand console navigation bar.
Click on the 'Activate APIs and services' button to search for and activate the Google Maps API.
Once the API has been activated, click on the 'Create a key' button to create a new API key.
Set the appropriate key restrictions according to the API's intended use. (restrict to sub-domain url)
Click on the 'Create' button to create the API key. You'll need to wait until you've defined the sub-domain url to finalize the process.
Three additional important points:
In the Application restrictions section: select 'websites' and add the URL of the site (this allows the key to be used only for the site).
The APIs must be enabled (in 'API restrictions', see screenshot): Maps Javascript, Maps Static, Places, Geocoding, Directions.
This is a monthly subscription, so you'll see the cost month by month.