How do I properly level the washer during installation?

Check levelness by placing a level on the top edge of the washer, first side to side, then front to back. If the washer is not level, prop the front with a wood block and adjust the feet as necessary, then prop the back and adjust; repeat until the washer is level and all four feet are stable on the floor. After leveling, use a 17 mm open-end wrench to tighten the nuts on all four feet tightly against the washer cabinet, as loose nuts can cause vibration. (Page 10)

From the official Whirlpool WFW9400SW00 Washing Machine manual, page 10

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