One very useful approach is to transcribe videos from the Internet. It is full of instructive and non-instructive beatbox videos, and you can learn from each one. Actually, the ones which aren’t superprofessional are easier to capture and transcribe, and it is a really good exercise to later understand the more complex ones. YouTube has tools to change the quality and the speed of the playback, which comes in handy.
Every beatboxer will develop his/her own language or repertoire of single sounds, and later beats and phrases, with which the whole show is then built up. Each single element requires a lot of training, dedication, and time. Later, it will seem improvised, but this is not completely true as big parts are fixed choreographies of the tongue and lips. Exchanging one sound of the choreography might take a while as not only will the sound itself change, but also the alignment with those before and after it. Like speech does with plosive consonants, beatboxing will use alternating points of articulation to avoid tiredness and cramps.
One approach to build up the beat could be as follows: First the columns of a beat are found, e.g. {b} (for bass drum), {pf} (for snare), then I add {t} (hi-hat), then other sounds like {k} to fill the gaps in between until I have something which sound like a grooving beat. After that, I can think about variations of that beat, like adding a special upbeat to the repetition, fill-ins, or a wild ending.
As far as my impressions are until now, the average beatboxer will use more or less exactly this buildup to show off the beat and what he/she can do with it. When finished, he/she will jump to the next beat, which will be as different as possible. Often the whole beat from the buildup to the connection to the next beat takes maximum 20 seconds.
Staying on one beat and varying the musical material seems not so interesting. It might include too many (boring) repetitions of the same beat, or the variations would be too little. Another explanation would be:
a) Using the same points of articulation is tiring for the muscles. The beats might get inaccurate, and this is just about the worst thing that can happen to beatboxers, who are normally supernaturally mechanical and groovy.
b) breath. Changing the beat allows you to sneak in a good breath as the rhythm is broken for a second. Normally, the beatboxer will breathe in with the articulation of certain sounds produced by inhaling, or take in air very, very quickly and in rhythm. On the other hand, he/she uses a lot of air on certain sounds, or, worse, one has to hold back air or can’t breathe out properly. Both lead very quickly to exhaustion, which would be totally acceptable if there wasn’t the danger of hearing your exhaustion through the high amplification of the microphone.
All these matters are good to keep in mind if you are a composer intending to write for beatboxing as these practical issues will come up when one tries to bring the writing to life.
Building up a basic beat