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Keep both feet firmly on the ground.
Literal
Both-feet [object-を] firmly [adverbial-と] ground [to-に] attach-keep [command-なさい].
~ていなさい combines ~ている (ongoing state) with なさい (polite imperative) to mean 'stay in the state of doing X.' しっかりと (firmly) uses the adverbial と. The sentence works both literally (physical balance) and as metaphorical advice to stay grounded.