How do you accord a composite avoir verb with a noun?

When according composite avoir verbs with nouns (most commonly passé composé), the verb may sometimes be accorded with the noun which is being acted on (the Compliment d'Object Direct—COD). This accord only occurs when the COD comes before the verb in the sentence. Be very careful here: while adjectives are accorded with their corresponding nouns, avoir verbs are never accorded with the subject, only with the COD (a type of object) when it arrives before the verb. Let's look at an example. In the phrase, "J'ai mangé la pomme," the verb (avoir mangé) is not accorded as the COD (la pomme) arrives after the verb. However, if this were to be rephrased, "La pomme que j'ai mangée," the verb would then arrive after the COD, requiring an accord with the feminine singular COD. Again, avoir verbs are never accorded with the subject (je) but rather the COD (la pomme) (when the COD comes before the verb). 

Also be careful that the COD is in fact being directly acted upon by the verb. If it is not, it is a Compliment d'Object Indirect and the verb will not be accorded with it. The easiest way to tell the difference is after saying the verb in the sentence, ask: 'Who?' or 'What?' In the example above, you could ask: 'J'ai mangé quoi?' The answer is 'la pomme.' La pomme is thus a COD. Dissimilarly, if one were to ask the same question after the sentence, "La personne à qui j'ai parlé," the question would not make sense. 'J'ai parlé qui?' (I spoke who?) whereas the proper question would be "J'ai parlé à qui?" (I spoke to whom?). The need for the 'à' signals that the object (la personne) is not being directly, but rather indirectly, acted upon by the verb (avoir parlé), making it a COI rather than a COD. Thus, the verb is not accorded with the noun in this case