Customizing Google Maps Clusters

I’m a beginner web developer who is experimenting with the Google Maps API for the first time. After some trial and error I was able to get map clusters working on my project but am struggling to understand how to customize them to match the aesthetics of my site. The documentation says to “Customize the cluster icon through the renderer interface” but clicking the provide link doesn’t provide any steps on that process.

Thanks in advance! I greatly appreciate any help or advice.