Identify the job: spreading on toast, creaming in cookies, or greasing/pan-release for cooking.
Choose the closest substitute from the table: butter for dairy recipes, plant-based butter for similar creaming, and shortening/lard for flakiness.
Use the listed ratio; if the substitute is harder/less water, warm it slightly so it creams evenly.
For salted margarine, match saltiness by using salted butter or adding a small pinch adjustment.
If the final texture seems dry or crumbly, increase moisture slightly (e.g., add 1–2 teaspoons water or milk to the batter) and re-mix.