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.