Answer:
Detrimental is an adjective used to describe something that is damaging, or harmful. The proper way to use it in a sentence would be when describing an event, or item, or activity (etc.) that will, can or is currently damaging another object or person. For example, saying "Smoking is detrimental to your health," would be one way to use it.
In this case, smoking would be the activity that you are describing as detrimental and your health would be the other "object" of which it is detrimental to.