Decide what you’re making: quick sauté/wilt (choose spinach) or longer braise/simmer (choose kale/collards/mustard).
Use the listed 1:1 ratios; if the substitute is wetter (spinach), dry it well or it will thin your sauce.
Prepare texture: trim tough ribs on kale/collards, and slice thicker greens so they cook at the same pace as chard.
Adjust timing: cook tougher greens 2–6 minutes longer depending on leaf thickness; add quicker greens at the end.
Taste-finish: add acid (lemon/vinegar) to brighten peppery swaps, or a pinch of salt to deepen mild ones.