There are two types of gifts that another user can get you.
Free Gifts
Free gifts will appear in your inbox, and you can press collect to put the items onto your account.
Purchased Gifts
To redeem a purchased gift, please follow these steps:
When you get sent a gift, you will get a message in your private chat with the gifter.
Navigate to the store where you normally purchase your diamonds.
Tap on the present icon.
In the redemption window, enter your code and press the ‘Redeem’ button.
Once you have done this, the code will disappear from your conversation and you will not be able to redeem it again.