Denouncing a Lord is a quest given by one vassal against another. All you have to do is ask a lord if there's a way you can advance your standings in the realm together while they are at a castle, town or on the Overland map by themselves. If there are other lords nearby, they will tell you: Perhaps we can discuss this in a more private setting. Due to the highly disproportionate exchange in relation, this quest is not advisable if you wish to be on relatively good terms with everyone.