Idea: Crossover Curtain Fic?

I had an idea for a Curtain Fic that is also a crossover of Code Geass and Mafia II. The basic idea is that C.C. and Vito Scaletta go to Pizza Hut. It's supposed to be funny and cute, and I may have it go past a one-shot.

But how should I approach this? I'm fairly new at actually writing fanfics. Yeah, it is a simple curtain fic about C.C. and Vito Scaletta going to Pizza Hut, but are there any pitfalls to avoid, if any?

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