Identify your spinach job: wilting for quick sauté, blending into sauce, or adding bulk to soups/stews.
Choose the closest match from the list: for fast-wilt dishes, pick kale (baby) or Swiss chard; for convenience, pick thawed frozen spinach.
Match quantity using the listed ratio, then adjust for intensity: if your substitute is stronger (arugula/broccoli rabe), start with less.
Cook the substitute according to leaf toughness: sauté fast for tender greens, and simmer/long-sauté collards or rapini until tender.
Taste and correct texture: if watery, squeeze or cook longer; if bland, add salt and a splash of acid (lemon/vinegar) near the end.