Decide your dish moisture: salads need a dry crunch, while soups need fast-added crunch.
Choose a substitute from the table by how dry you want it (toasted crumbs/chips/crackers).
Use the listed ratio and add timing: top salads right before eating; add soup toppings in the last 1-2 minutes (or after ladling).
If your substitute seems too dry, mist with oil or broth; if too soft, toast 2-3 minutes or add later.
Taste for seasoning: croutons are often garlicky/salty, so add salt, garlic powder, and pepper to match.