The patient is taught how to relax their muscles completely. This is because a relaxed state is uncompatitable with anxiety.
The therapist and patient together construct a de-sensitisation hierarchy - a series of imagined scenes, each one causing a little more anxiety than the previous one.
The patient gradually works their way through de-sensitisation hierarchy, visualising each anxiety-evoking event while engaging in the competing relaxing response.
Once the patient has mastered one step in the hierarchy, i.e. they can remain relaxed whilst imagining it, they are ready to move onto the next.
The patient eventually masters the feared situation that cause them to seek help in the first place.
Problem - fear of spiders.
Result - overcome the fear and feels relaxed in spiders presence.