Beatboxers use a microphone to help hear the beats more clearly.  They cup their hands around the top of the microphone in different ways, the idea being that the maximum amount of sounds are collected by the microphone.  There are three ways to hold the mic; closed, open, and two mics.   There is no right or wrong way to hold the microphone.  The technique comes through experimentation but be aware of covering up the mic completely because this may make the microphone ineffective or create feedback.  

The most often used method for beatboxers is closed clasp.  In this technique, nasal bass sounds are directed into the microphone.  The fingers are wrapped tightly around the microphone for maximum bass.  

In the open clasp, the beatboxer is not bothered with capturing the nasal sounds.  The fingers are spread more, leaving gaps in between the fingers.   

To practise your beatboxing without a microphone, you can cup your hands around your mouth to collect the maximum amount of sounds, until you are confident.