。
Write that down so you don't forget.
Literal
Forget-not [so-that-ように] that [object-を] note-down-in-advance [imperative-なさい].
~ておく (do in preparation) + なさい (soft imperative). なさい is the gentle imperative typically used by parents, teachers, or anyone in a caring authority role — less harsh than the bare imperative 書き留めておけ. Combined with ~ないように, this is a practical, parental-sounding directive.