If you are already subscribed to the creator:
On the Creator's Club page, tap the button "Tip".
You can select predefined amounts or enter a Custom one.
You have the option to add a message with your tip
Confirm the payment method and Validate the payment!
NOTE: If you're not already subscribed to a Creator, you can still tip them. However, tipping a Creator will not unlock their content. You will still have to buy their subscription.