Here are the steps for setting Unicorn up on a preview/test theme so you can see how everything functions without activating the script on your live theme.
Step 1: Duplicate your live theme
Then head back to the Unicorn app and follow the cart onboarding steps.
You won't be installing the cart theme block and script tag on your live site, which Unicorn automatically links to with the buttons. Instead, you need to install them on your draft theme.
In the draft theme, click 'customize' and activate the theme block and script tag there.
Note: In your cart onboarding progress, the app won't show these steps as completed, because they are not active in the live theme.
Set the cart up for testing and then make the cart 'Active'.
You can then go to preview your test theme and preview the Unicorn cart in there.
Note: the 'cart preview' button won't work as this shows your live site - so to preview the cart you need to make it active on the test theme and then preview the theme itself.