Custom Quizzes has been renamed to Create Challenge. It is a Pro feature to customize a quiz.
Start with navigating to a Quiz you would want to customize, for instance - https://www.geoguessr.com/vgp/3003
Click on Create Challenge and then you can start with setting a Title and Description to your quiz.
Chose to have a Public leaderboard and if you want to Anonymize nicknames added by the ones who play it.
You can select/deselect options on the quiz as well as chose game mode. Please note that its not possible currently to change the game mode once the quiz is created.
At the bottom you can add your own locations to a map. Please note that if you add multiple locations they cannot be called exactly the same.