Identify your recipe type: baking/cake frosting vs glaze/icing, because alcohol-based options evaporate differently than juice.
Start with the table ratio for your chosen substitute (extract-like aroma first; pure tang last).
Mix thoroughly: for zest-based swaps, stir and let sit 2–5 minutes; for oil/citric acid, dilute into a small amount of liquid or sugar first.
Adjust in small steps: if the flavor is flat, add a pinch more zest or 1–2 teaspoons more lemon juice concentrate per batch; if too sharp, balance with a bit more sugar.
Do a quick taste test of the liquid component (or batter before baking) so you can correct aroma/tang before it sets.