Identify your cooking method: quick stir-fry needs stem-and-tip timing; longer braises can forgive leaf-only substitutes.
Choose the closest texture from the table (stems first vs leafy first) and use the listed ratio.
Cut stems to similar thickness: thinner stems = faster finish; thicker stems = extend heat by 30–90 seconds.
Adjust to bitterness/mildness: blanch rapini briefly; massage kale; keep spinach as a last-add.
Finish with sauce: toss 10–30 seconds at the end so the substitute coats without overcooking.