Concrete sculpture base?

I'd like to make some concrete sculpture bases, about a foot deep by a foot or two wide/long. I've made a mold box out of some scrap laminate board, to be placed ontop of a sealed board. Was hoping I could ask a couple questions from someone far wiser in these sorts of things before I make my first pour:
Will I need anything other than oil clay tamped around the edges to hold the cement inside?
Do I use just regular sand mix cement? Keep it covered/damp how long? How do I make the top flat and smooth? How soon can I take the mold walls off? Thanks for any questions you want to answer (and I apologize for the number of same).
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