Decide your use: baking/quick breads usually want neutral flavor, while frying benefits from higher heat stability.
Pick your substitute from the table below and start with the listed substitution ratio.
If your substitute is thicker (like shortening), melt it or warm it until pourable before mixing into batter.
If your substitute has noticeable flavor (like olive oil), reduce its impact by using it in recipes where the other flavors are bold (or choose extra-light).
Taste-test the batter or sauce briefly (uncooked batter where appropriate) and adjust with a tiny splash of water or a pinch of salt if needed for balance.