Isolate yourself in a quiet room. If you have a songwriting partner, have him join you.
Take out your pen and notepad and write the title of your song at the top of your first page. Scribble some lyrics on the page if you already have words in mind.
Write down words that rhyme with the title of your song.
Create some nursery rhymes using the rhyming words you wrote down. Recite aloud each and every word you wrote down.
Decide which rhymes you like best and form a sentence. Write this sentence down on your notepad.
Turn on an instrumental hip-hop track and recite your sentence aloud multiple times over the beat. If you are satisfied with the synergy of the track and your words, then you have a good hip-hop hook.