Professional dancers usually count out songs by eight counts to help ensure dancers sync with each other and the choreographed dance syncs with the music. Feel the pulse of the beat naturally and count from one to eight throughout the song, making a notation in the lyrics where the first, middle, and last counts fall. After you've imagined out your routine and gotten a feel for the music, there are likely a few dominant moves that you feel must be incorporated into your hip hop routine. Use these as a focal point. Distribute these moves throughout your song at appropriate places, and then come up with transitions that work into or out of your main moves.  Use the transitions between moves to build or release tension. Coordinate your main moves with your counts so that you clearly know where in the song your dancers are performing specific moves. Dancing is a strenuous activity. It's a lot easier to sit down and write ideas, but at some point your dancers are probably going to need a place in the routine to catch their breath. Insert pauses or slower parts of your routine between, before, or after high energy parts of your choreography so your dancers aren't left gasping. Now that you have a fairly full image of the story you're trying to tell through the motions of you choreography, the main choreographic moves you want to hit, and the transitions linking these together, you'll need to write it all down. Be sure you include counts so that you can coordinate choreography practice and the moves being practiced more easily.

Summary:
Count out the song and your moves. Break your routine into main moves. Remember to give your dancers a breather. Write up your moves and counts.