Script writing is a very difficult topic to discuss because I think my views sort of go against what most people believe. But in any case I'll throw it out here.
I don't think you should write a script first. There I said it. I'm not going to apologize, because I truly believe that.
Step one is not write a script. Step one is PLAY. Yes, that's right, play. Explore the material you have worked on, try different approaches to presenting it, just throw out random ideas and thoughts. It's okay to hmmm and hawww your way through a new piece. In fact it's important that you do that! You need try presenting it as a little bit of nonsense, and try presenting it as the greatest trick of all time. Do it quick and snappy, and try drawing it out and making it longer and dramatic. All of this is okay!
In any creative art there needs to be room for exploration, playing, and general fun. It's not always fun, but I think this aspect of it should have a certain playfulness to it. You don't have to know how a trick is going to be presented.
You write a script once you've made some sort of educated decision on the direction you want to go or the way you want to play things off.
It's okay to not have a script. In fact it's a good idea to NOT have any script for a while! Know your effect, know your method, execute proficiently. Then present it in whatever way you feel it may go over best. Eventually one presentation will start to grow on you and THAT is the time that you can start sitting down, with a clear idea of where you want the script to go and you can script it out. Until then, keep playing, keep enjoying, exploring, and challenging yourself to come up with the best way for you to present your magic.
Play now, script later!