Get a Google API key
Visit console.developers.google.com
Sign in with your Google account if you are not already signed in.

If you have any existing projects, you should probably create a new one for this. Let's keep things clean.

Visit https://console.cloud.google.com/cloud-resource-manager

Click 'Create Project'
Type 'Google Reviews' as the project name.
Click 'Create'
On the right hand side, you should see a notification.
Click 'Select Project'
In the centre of the dashboard, there should be an APIs card.
Click 'Go to APIs overview'
Click 'Enable APIs and services'
Search for/Select the 'Places API' and click the card.
If it isn't already enabled from a previous project, Click 'Enable'.

This will probably take you to your Google Maps Platform API list where you will see which of the Maps APIs are enabled and disabled. There will be at least one enabled - the Places API.

Search for the Javascript Maps API and enable this as well.

Find and click on 'Credentials' on the left hand side.
Click 'Create Credentials' at the top > API Key.
In the window which appears, click 'Restrict the key'.
You will be taken to the key editor.
Rename the key - I'll call this 'google_review_places_api_key'.
Restrict the applications to 'http referrer' and add the website match as a 'New Item'.
I used *.computing.cafe/* to restrict the key to this website only.
Restrict the key and only allow it to be used in the Places API.
Save the changes to the key.