Picture visual images in your head of what the choreography should look like as you listen to a song or piece of music.
Name the steps of the dance movements so that you can write them down. Each movement should have its own name. These can be anything you want to call them.
Sketch visual interpretations of the dance movements next to the names you have given them. The sketches can be in any style as long as you understand them.
Mark moments in the song, or music, where you want certain dance moves to occur. Depending on the type of music you are writing to, there will be different ways of marking the movements in the piece. Classical, jazz and pop music all have different structures. Modern dance often isn't even performed to music, or is accompanied by spoken word. In this case the sketches of the dance movements alone would be arranged on paper. This is in accordance with the fact that "written" choreography is a mix of sketches and ideas.
Build sequences on paper that correspond to your vision of the choreography. The movements may be wildly divergent, or narrative in nature, depending on the music. Write down the names of the moves, or sketch the image of the dance moves next to your markings for the song movements. In the case of "writing" choreography to a non-musical piece, or minimalist piece such as spoken word, the document would look more like a shorthand drawing. Imagine it being arranged like a storyboard of dance movements.