Read the original question you want to answer as a complete sentence. Rewrite the question in the form of a statement, starting with the subject. For example, to answer "What did the dog want to do after its nap?" as a complete sentence, begin with the subject, "The dog."
Find what the subject is doing or wants to do and add that to the sentence. The sentence now reads, "The dog wanted to _____________."
Check if the sentence makes sense by itself and if it tells you what the subject did. If not, it's still incomplete.
End with the answer to the question (the predicate), keeping the initial question intact. For example, "play after its nap." The complete sentence now reads: "The dog wanted to play after its nap."
Include the subject and predicate to make a sentence complete. In the sentence, "I love ice cream," the sentence is short but includes a subject -- I -- and predicate -- love ice cream. This indicates the sentence is complete.