Practice is never a bad thing. Unless it becomes the only thing, in which case it is a bad thing.
Some people have good luck with jamming or noodling, and happening upon riffs and cobbling stuff together and calling it a song.
I'm not one of them.
However, I'm in the minority I'm sure. Most rock bands at least seem to write songs this way, with the instruments working up music and a singer sticking lyrics on top.
I prefer a more integrated, organic approach. I write lyrics along with melody, and do so without picking up an instrument. Once I have the whole thing written, a capella as it were, I pick up a guitar and figure out the chord progression, and, if necessary, melody notes. By writing this way you don't have to know much theory, if it works as a song first, with no instruments, it'll be fine once you add them. You do have to know your instrument well enough to figure out chords for the a capella song, however. An upside of writing with voice only however, is that one tends to write only what one can sing, rather than something that can be outside the range of your singing voice.
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