Identify what the recipe needs most: juice (drinks, curds, glazes) or zest (baking, aroma-heavy desserts).
Start with the listed ratio (most swaps use 1:1); if using lemon, begin with the lower ratio and build up.
If your substitute is less aromatic (like orange juice), add a small amount of fine zest to restore the fragrance.
Taste the mixture once mixed into the base; adjust sweetness with sugar/honey and reduce sharpness by adding a touch more fruit pulp/juice.
For zest-heavy recipes, use only the colored zest (avoid pith) to prevent bitterness.