Match the substitute to your use: for light, neutral creaminess choose half-and-half or the milk+butter blend.
Use the listed ratio (1:1) and mix off-heat if the substitute is thicker (evaporated milk, coconut milk).
If your substitute is prone to curdling (Greek yogurt), temper first: whisk in a splash of warm liquid, then add to the pot gradually.
For baking, start with the same ratio and check batter/sauce thickness; add 1–2 teaspoon water or milk if it looks too thick.
Taste and adjust salt and sweetness after cooking, since some swaps (evaporated milk, coconut milk) shift flavor.