There are different instructions depending on which platform you're purchasing this feature for. Please find the appropriate platform below. Note that Unlimited Turns is a non-consumable one-time purchase; you keep the benefits even when you get to the Game Over state and have to start a new game!
Mobile Game - iOS, Google Play (most Android devices), Amazon (Kindle Fire)
First, play through the tutorial if you haven't done so already. Then follow the screenshots below. (The appearance of these buttons has changed slightly as the game has evolved, so look below for the screenshots most representative of your game.)
Steam, Amazon, GOG, or other version installed directly on your PC / Mac
This was included when you purchased the game. You don't have to purchase it separately.