A complete predicate has a transitive verb (a verb that takes an object). A transitive verb is incomplete without an object to act on. The predicate is the part of the sentence that includes the transitive verb as well as the object of the verb, any adverbs that describe the verb and any adjectives that describe the object:
Sally hit the red ball hard.
The predicate is ‘hit the red ball hard’. ‘Hit’ is a transitive verb and it acts on the object of the sentence; the ‘ball’. ‘Red’ is an adjective that describes the object, and ‘hard’ is an adverb that describes the verb ‘hit’. So they all get included in the predicate.