In the game lobby, select the "VS Friend" game mode. After that, a "Create" button will appear in the lower right corner of the lobby. When you tap it, a unique code will be generated, which your friend can use to join the duel.

If your friend is already on your friends list, you can invite them to a duel using the quick "Invite to Duel" button in your friends list, which will send them a notification with an offer to duel. It's important to note that the notification will only arrive if your friend is online and in the game lobby.