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Walk in line properly all the way to the doorway.
Literal
Line [in-に] form-ing neatly doorway [until-まで] walk [imperative-なさい].
列になる is an idiomatic 'to form a line / be in a row.' The て-form 列になって modifies the walking adverbially — 'walking while in formation.' きちんと ('properly, neatly, correctly') is a common adverb for orderly behavior. 戸口 ('doorway, entrance') takes まで for the endpoint. 歩きなさい is the ~なさい polite imperative.