Acquire an actual metronome, download a metronome app, use a simple beat, or even just have a friend slowly, consistently, clap for you. Start by freestyling over the metronome with one syllable per beat. The object is to keep it simple and stay on the beat. As you progress, you can speed up a little, and eventually try doubles (two syllables per beat) and triplets (three syllables per beat). Then you can start to play with it. Try speaking on the offbeat (between the beats) but be consistent. Pay attention to the rhythm, not your own voice. If it starts to get boring, that’s good! You’re training your subconscious to just listen and develop an innate sense of rhythm. This is how drummers and other musicians practice their sense of tempo and learn to keep time.
Some people have a stronger innate sense of rhythm than others. They’ll really need this practice, but everyone can benefit from it. Some people are paying so much attention to their voice and what they are trying to say that it seems like they aren’t listening to the beat at all. This exercise can improve their flow dramatically.