Answer:
While some might argue against this, it is no less true: poetry must be inspired. If that "just because" reason or event or observation is what inspired you, then ride the emotion--poetry is about the emotion, what you feel--and get it down on paper. Once there, you can develop it further, or edit it to work better for what you intend.
So, "How"" is best answered by just do it. Write it down and keep going.
Of course it helps tremendously to be familiar with forms of poetry, and past examples of the art. The more you know, the more you can do.