we usually do it like this: we have a person who writes lyrics in the band (no - in our case its not the singer). So we have a bunch of lyrics to choose from.
we enter the rehersal room and someone is like "look i got this and that". then we jam along to it. usually after 1-2 hours we got at least 3 parts/riffs and then we start to construct a song from it.
our singer goes through the lyrics and is like "hmm.. yea - this text will fit". then he starts to come up with vocal lines and we try to come up with a structure for the song which is not too difficult if you have a few riffs and the lyrics. so basically we have an output of at least one song per rehersal.