Identify your recipe role for skim milk: main liquid (baking batter), acid + tenderness (quick breads), or smoothing (sauces).
Choose the closest match from the table (dairy low-fat diluted first; then unsweetened soy/oat; then yogurt if you can thin).
Use the listed ratio. If your substitute is thicker than skim, thin with water in small splashes until it pours similarly.
For yogurt-based swaps, add to room-temp batter and mix well to reduce curdling; bake per your recipe timing.
Taste-check once baked or simmered: if it tastes flat, add a pinch of salt; if it seems too heavy, thin slightly next time.