Pre planning for large piece for a school assignment

So I was wondering if I could be instructed on the preparation for a piece with lots of instruments. I'm gonna do a full orchestra piece but I don't know where to start. 

Should I plan melody, the chords, the shape or the form first? And in what order do you suggest?

The piece is gonna be at most like 3 minutes long because I have to present it with minimum embarrassment as possible.

If it's only a three-minute piece and you've written things that length before, I'd do it in whatever order you usually do it.  Don't let yourself get intimidated.  The only thing that changes here is you are adding some depth to your orchestration experience.  Form, melody, and chord structure work the same way they always did, so as long as you haven't had any problems with the way you usually do it, do it the usual way.  Have fun!  They call it "playing" music for a reason.  This is an opportunity to play.  With a whole orchestra!

If it's less intimidating, or easier to read at a glance, you could do your initial planning as a piano score with little notes to self about instruments when they occur to you, and then go back and orchestrate it and make a proper full score.  

The main other thing to plan for is timbre. What instrument do you want to make which sound where? Is it an extended technique? How does it blend with the other instruments there?
Things like that.

Maybe plan out a little bit of the end, middle and beginning. then fill in the rest?