Hip-hop dominates popular culture today. The primary voice of this ubiquitous art form speaks in rhyme and rhythm. In rap, as in music in general, bars (measures) break a song into bite-sized bits. Each bar contains a certain number of beats. In hip-hop, four beats per bar is most common. All the instruments in a song follow this pattern, and an MC's voice is no exception. The best rappers fit their rhymes seamlessly into the bars of their songs. Their voices function as another instrument. Aspiring rhyme-smiths may have the ability to write intricate, sophisticated lyrics, but a song is about the whole package. If the words don't match the music rhythmically, the song's bound to flop.
Find an instrumental beat track to work with. Many websites offer downloadable beats for free. Having a audible rhythm going while you write helps ensure your lines stay consistent. Don't rely on a beat in your head, because interruptions or exhaustion can make the beat inconsistent. This makes your rhymes inconsistent, too.
Count the beat out loud. The heavy usage of drums in hip-hop makes counting the beat fairly simple. You're looking for four regular counts. Usually a hit on a snare drum or hi-hat denotes the two and four beats. When you hear the hit, start counting: one-hit-three-hit. Think of the hits as punctuation. The hit on two functions like a comma, and the hit on four functions like a period. It's important to be able to count the beats, because even though the majority of rap songs use the four-beat formula, the speed at which they're delivered may vary.
Begin writing rhymes. Start simple. A common strategy is to write in two-bar loops, similar to rhyming couplets. Bar one rhymes with bar two, bar three rhymes with bar four, and so on. Each couplet often represents a complete thought. For example, 2Pac begins his song "Dear Mama" like this: "When I was young, me and my mama had beef./ Seventeen years old, kicked out on the street./ Though back at the time, I never thought I'd see her face,/ Ain't no woman alive that can take my mama's place." He uses four bars and two ending rhymes. Each couplet constitutes a complete sentence or thought. Notice he also utilizes the midbar breaks.
When writing your own couplets, start by putting down a series of paired end words like bird, word, hazy, lazy. Each end word matches up with a four-beat drum hit. Complete the lines so they fit in the rest of the bar ("Looking out the window, I saw a funny bird./ Guacamole tuna fish, listen to my words/ Rain and clouds sure make things look hazy/ All this fog outside just makes me feel lazy"). They don't have to make sense at first. Just get used to putting words into a rhythm. Make sure you rap the words as you write them, so you hear the way the spoken rhythm fits with the beat. Rapping allows a lot of room for slant rhyme, so pay attention to how words sound, or how they are pronounced, not just how they look on a page. For example, "still" and "steal" don't rhyme on paper, but they sound like they do if spoken with the right inflection.
Play with the rhyme scheme. Once you get the hang of simple end rhymes, try adding internal rhyme. Eminem is a master of internal rhyme. He displays this talent in his song "Lose Yourself": "His palms are sweaty, knees weak, arms are heavy/ There's vomit on his sweater already, mom's spaghetti." In two bars he rhymes the "-eady" sound five times and "-om" twice. Squeezing seven rhymes into two coherent lines comes with a lot of practice. A good way to extend your rhyme library is to try rhyming multiple words with one. For example, rhyme "incomplete" with "win some feet." Try to come up with as many variations as you can. Try playing with the end rhyme pattern, too. Many rap songs continue with the same end rhyme for many bars.
Finally, play with your vocal inflection and the way you speak certain syllables. You can stretch a too-short line ("I want thiiiiiis to fit"), or mush a too long line ("so-I-spit-a-lot-faster-than-pigs-spin-over-pits") just by controlling your delivery. Keep in mind that listeners have to be able to understand you, so if you can't articulate, keep it simple.
Practice. Neither 2Pac nor Eminem achieved their superstar status without a lot of work. Both put a lot of time and effort into their rhymes. Make rhyming word lists, play with your delivery and record yourself. Often a verse you think sounds tight sounds a little different when played back. Think of verses while you work, in the car or during any downtime.