Nice to see you again! Hope you are enjoying your house still. I dont know that band or what chords they are playing. You can write a song with just a guitar and a vocal melody. Heck you can skip the singing entirely and just play a song on a guitar. It's a big giant puzzle to write a song and sometimes there is so much to it that it can get overwhelming, recording a song is even more to it cause then you will most likely be adding other instruments and that adds so many more choices. I like to just write a song on an acoustic first and add the bass lines and drum beats later. Or sometimes I come up with a bass line and try to find a guitar part that compliments it later to add to the song. Its challenging but rewarding if it turns out like you imagined