Identify how the recipe uses crème de menthe: color/flavor only (syrupy) vs liqueur function (alcohol lift).
Pick the closest match: mint simple syrup (nonalcoholic), peppermint extract + simple syrup (most control), or green chartreuse (herbal punch).
Measure using the ratio target; if your substitute is homemade or concentrated, scale in small steps and taste after mixing.
If the substitute is less sweet or less minty than crème de menthe, add extra simple syrup (for sweetness) or a few drops of extract (for mint).
For drinks, chill and then re-taste; for desserts, fold in gently and let flavors sit for 10 minutes before final adjustment.