there's no right or wrong answer when it come to writing music. i play guitar but i sometimes start writing a song with drum loop that inspire a cool riff or chord progression. then maybe i record a bass or a vocal line. i usually write a lyrics together with vocal line. i dont do lalalala nanan songs. usually.
but as long as you can write a great song i don't think you should worry about the way you write it.
if you want to get deeper into songwriting you should check some Pat pattison videos on youtube. really cool guy.