Decide whether you need a blend (most baking) or a single-spice rescue (cinnamon-only).
Replace apple pie spice using the listed ratio (use 1:1 for blends; use the ¾:1 for cinnamon-only).
Mix the substitute into dry ingredients before adding liquids, or stir into melted butter/syrup if your recipe uses a spiced glaze.
If the substitute includes ginger/allspice and your apples taste tart, you can slightly increase the sweet element (like sugar) to rebalance.
Taste once the filling is warmed (or right before baking if your recipe allows). Adjust with a pinch more cinnamon or nutmeg rather than adding lots of spice.