Sometimes there are specific traumas that are an obvious cause of phobias and sometimes not. For example, we may have had a close call with drowning in a bathtub as a toddler, and thereafter have a fear of water. Or there may be no obvious cause like this.
Either way, we work with it the same way we work with anything else – somatically! What are the sensations in the body happening when the phobia rears its head? That’s the first thing to find out. Then we work with that experience at the body level in the ways that you are learning.