Firstly it is important to understand that Wicca is in no way related to the beliefs and practices of the Celts. This is an excellent article on that topic: http://www.clannada.org/wicca.php
That being said, most who practice something they refer to as "Celtic Wicca" mean that they are practicing Wicca that includes Gods and Goddesses drawn from one or more of the Celtic pantheons. So if you read on the beliefs and practices of Wicca and honor a Celtic God and a Celtic Goddess then you are presumably practicing "Celtic Wicca".
You could also read up on the practices of modern Druidry, which is very loosely based on Celtic lore and then include those in your Wiccan practice.
Remember that we know next to nothing about the religion of the many Celtic tribes other than the names of some of their Deities as the Celts had no written language and did not pass on their tales. What passes for Celtic beliefs these days almost certainly is nothing like what the ancient Celts believed.