The majority of songs we play are originals. These form in several ways, they are...
The Singer (guitarist/banjo whacker) says "I've come up with a song", He has lyrics & a chord arrangement. We all write our own parts around what he comes with, sometimes adapting what he has done to suit what we want.
I come in with a bassline/ chord arrangement/ series of riffs/ melody and we work on it from there. We always let the singer write the lyrics.
We jam & something evolves from something one of us are doing, quite often something turning out to be sheer genius.
Problem I have is the lead guitarist can't remember sh!t & it can be quite anoying. especially if he's come up with a good hookline for in a song & then a month down the line he never plays it again! Other problem is constant noodling! Especially when there's singing going on.
Whenever we cover something else, we do our version of it. As if someone said "there's some lyrics & a chord arrangement, have fun".