The official how-to is a bit brief, so let me expand it a bit.
The first step is to sign-in into your Google Analytics account, go to Admin and click the Create Property button.
Next step is to fill in a simple form.
I’ve put in the name I can see in the Browser when I visit my itch.io page.
URL is pretty clear, it’s YOURPAGE.itch.io.
Industry Category “Arts and Entertainment”, but I guess you can put any category depending on what’s your itch.io page all about.
Your Time Zone, duh.
Now, click the Get Tracking ID button.
Now, open up another browser window, visit your itch.io page and wait a few seconds.
In your Google Analytics, you should see a number of Active Users.