Identify what you need from rye bread: tangy flavor, dense chew, or baking structure (e.g., stuffing, crust base, croutons).
Pick the substitute that matches that role from the rye bread substitute table, starting with pumpernickel for closest rye-like character.
Use the listed ratio. If your substitute is softer (like oat bread), toast or bake it a few extra minutes to firm up the crumb.
For baked uses (croutons/crumbs), cube/slice to the same size you would use for rye so texture browns evenly.
Taste after assembly and adjust: add acidity (lemon/vinegar) if the flavor feels flat compared with rye.