Summarize the following:
Singing and beatboxing at the same time may seem like an impossible task (especially at first). But it's actually quite easy. Below is a working sample that will help you get started. You can use this basic technique and later adapt it to any song. (b)if your (pff)mother (b)(b)on(b)(pff)ly knew(b)knew(pff)  ("If Your Mother Only Knew" by Rahzel). Listen to the song you want to beatbox to a few times to find out where the beat goes. In the example above, the beats are marked out. This will help you get comfortable with the song. Most songs will have the beat in front of the words. In this case:  "If" - Since the word "if" in our example starts with a vowel, it is easy to fit in the bass just before it, as though you were saying "bif." Note however, that the "b" must be low and if necessary, separate the beats from the words a little when you first start. "Mother" - The word "mother" starts with a consonant. In this case, you could drop the "m" and substitute it with the "pff" since they sound quite close when said together quickly. Or, you could stagger the word just a little so that the beat comes first, and the lyric slightly delayed. If you choose the first, you will end up singing "pffother." Notice that your top teeth contact your lower lip, which is what creates the m-like sound. If you can manipulate this, it will sound a lot better. "On" - For the double beat on "on," you can hum the pitch while doing "b-b-on," then come in straight away with "b pff-ly knew," all the while humming the pitch. For the "on," you might find that the sound breaks if you do the second bass beat. To remedy this, hum through your nose. This can be done simply by pushing the back of your tongue up to close off against your soft upper palate. This hum now comes out through your nose, and is not interrupted by what you do with your mouth. "Knew" - The word "knew" echoes and fades off. These steps can be adapted for any song with a beat. Keep practicing, with different songs and soon you will be able to ad-lib more easily.

Summary:
Sing and beatbox. Listen to songs. Sing the tune a few times with words. try to fit the beats with the lyrics. Adapt this skill.