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If you want to play, make sure to set the rules first.
Literal
Play-want [if-なら] properly rules [object-を] decide-[preparatory-ておき][command-なさい].
遊びたいなら uses the conditional なら 'if it's the case that.' ちゃんと means 'properly, correctly.' 決めておきなさい combines ~ておく (preparatory action) with ~なさい (mild command from a superior, often parent to child). The tone is authoritative but not harsh — a parent or teacher setting expectations.