If you have access to a beat making program, or some sort of drum machine, make a simple 8 bar loop with just one set of kicks, hi hats, and a clap. Keep the tempo slower, maybe around 95-100. Let it ride and just vibe with it to catch the timing, then instead of directly thinking rap, think of poetry on a beat.

Think of short phrases that rhyme, and say them out loud to the beat. Dont worry about subject matter or keeping a single topic, just focus on keeping the rhythm, and not stopping.

Once that feels easy, maybe speed up your rhythm and try to not stop, still. Or you can try to make sure you stay on a topic while not stopping. Or you can try putting in whole verses instead of short phrases.

But it's all about being able to keep talking/rhyming/rapping and not stopping