It's easy! In fact, there are a number of ways to compare a case on Polaris. One way is through the case detail page. You will find a button on the top displaying 'Compare +'. This will prompt you to start a comparison with the case you are currently on.
You can also compare cases while in the 'Case' tab. Find a checkmark in the lower right corner of the case card. By tapping on two different checkmarks on two different cases, a compare button will appear in the upper right corner. Tap on it, and just like that, you have created a case comparison!