Decide whether you’re making a cocktail or baking: cocktails need aroma plus sweetness, while baking needs flavor lift without thinning batter too much.
Start with the matching ratio from the list; for peach liqueur and apricot liqueur, begin at 1:1.
If your substitute is thicker (syrup/jam), thin it slightly with water or juice so it dissolves cleanly in the drink or glaze.
Tune the balance: add lemon juice for brightness or a touch more syrup if the swap tastes flat.
Taste after mixing (cocktails) or after combining (baking glaze/batter); adjust in small increments so the peach flavor stays in front.