Decide what you need Dijon for: tang (vinaigrette/sauce), emulsifying (mayonnaise-based dressings), or mild heat (marinade/roast).
Pick the closest match from the substitute list and start with the given ratio (usually 1:1).
Sharpen or round the flavor: add vinegar a few drops at a time for tang, or a tiny pinch of sugar/honey to tame bitterness.
Let the mixture sit 2–5 minutes (especially for dressings) so it emulsifies and tastes more like Dijon.
Taste and adjust salt/acid before adding to your final sauce or dressing.