Find your inspiration. Every musician is inspired by artists, such as poets or other musicians. Listen to some of the all-time hip hop greats, and pay special attention to their love songs. Ja Rule, Faboolus, and P. Diddy have written some of the best love raps; listen, learn, and then try writing some of your own.
Create a list of things you know to be true; this gives you a foundation of truth and sincerity from which you can build your love rap. What are things you "know to be true?" Acclaimed spoken word poet Sarah Kay asks this question to her students as they work on their love raps and poems. If you continue to suffer from writer's block, Kay recommends you make more lists: "Things I would give up for love," "Things I wish I had done by now," etc. Lists are a great way to organize your thoughts and get your rap going.
Write your love rap with a strong emphasis on rhyme. Choose the final word of the second line of a couplet. This makes rhyming much easier and more fluid. If you write the first line of your couplet first, you may be forced to rhyme with an awkward word.