Start with the closest flavor goal: more bitterness (Campari/Cynar) or more orange-bright sweetness (red vermouth/orange liqueur).
Use the listed ratio in place of Aperol; if the substitute is darker or sharper, dilute slightly (water/soda) instead of adding more sweetener.
For cocktails, taste the liquid base before building the spritz; adjust with a small splash of orange juice or simple syrup to match Aperol’s gentle sweetness.
For cooking, stir the substitute in during the final minutes so the bitterness and orange notes stay lively.
If your result tastes flat, add acidity (a few drops citrus or a tiny pinch of salt) rather than more sugar.