If you get a notification indicating you've received an item as a gift, congratulations! You now can decide whether to apply it to your own account or – should you not want it for yourself – gift it to another user. Assuming you've opted to do the former, however, simply follow these steps:
Open the Dashboard.
Click your Account avatar in the corner.
In the sidebar, click Inventory.
Locate the item you'd like to use and click its Manage icon.
Click Use under Actions to continue. Note that if the item can't be used on your account, this action won't be available (in which case, consider gifting it to another user ;)
Confirm that you want to do this by checking the confirmation checkbox.
Click Use Item
You're done! The item and its described effects should now be applied to your account.