aiyah... just bantai only la... if drummer starts a random drum, try to play something on the guitar if you're the guitarist according to his drum beats. if u all think it's cool, play again then record it down so u don't forget. well, at least that's what i do with my band. some1 will have to start, don't care who start 1st.

sometimes the bassist has a cool groove. then u just work from there on. sometimes the vocalist has some tunes in his head. again work from there on.

hope this helps. good luck!