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Make the most of your free time.
Literal
Free-time [object-を] fully use-[make-so-ようにする]-[soft-imperative-なさい].
An aphoristic piece of written advice using the ~ようにする pattern, which means 'make it so that' or 'try to make a habit of' — a softer, habitual framing than the more effortful ~ように努める. 十分に ('sufficiently, fully') is adverbial here and attaches to 利用する, not to 余暇.