You just talk to a beat, the flow is more of a personal thing, but you will develop it
Edit: Oh you want lyrics! just write it like a poem, also use this http://www.rhymezone.com Also, listen to a wide variety of rappers to get a sense of flow, very important. (no mainstream s***, look up dumbfoundead, Watsky, Breezy Lovejoy, Wax, and Mac Lethal
Say your words really fast to a rhythm and rhyme. Pretty easy actually unless your talking about copying someone then you look at the lyrics then you listen to the song while looking at the lyrics and try to say it at the same speed!!