Decide the job: cereal topping (needs light crunch) vs coating/breading (needs good clinging) vs baking (needs structure).
Choose the substitute based on texture: bran/rice for similar cereal crunch, panko for even breaded crunch, oats for body, crackers/cornbread crumbs for emergency structure.
Use the listed ratio; for coatings, match particle size (crush cereal/crackers, or pulse oats) so everything browns at the same rate.
If your substitute looks wet or clumps, toast it dry 2–4 minutes (or bake the coated item slightly longer) to restore crispness.
Taste and adjust: if the result tastes bland, increase seasoning in your binding (egg/marinade) rather than adding more crumbs.