It's quite simple. You discuss what should it should include. Like, you find the theme. Like "Sad Wasteland" Or "Joyful Forest". And just let your minds run on it. Kinda like "Hey what about *SOMETHINGSOMETHING*?" "Yeah, that works, but how about *Somethingsomething*."
You discuss it and refine it. Get two minds working on it.
Nice poem. Thanks for making this. I just got away from a fanfic in which Rarity invited her "dearest friends" to her new home in Spike's hometown, but failed to invite him for some unexplained reason. This went a short distance in restoring my good mood.