Lyric discussion by Schimmel 

Cover art for Once lyrics by Sophie Zelmani

I read it as a woman and a man getting together to make music, him being very focused on making music:

"Here babe Are the notes you must play This babe, is the procedure"

Although he's possibly enamored with her too, or at least they have the relation of him calling her babe. 'Once' would be about maybe just one kiss, or making love one single time to just "get it out of their systems".

He's telling her she needs self-control (although it's not always clear where the border goes between the singer and the character, since they sing a-cappella in the chorus which I'd interpret as only her thoughts otherwise):

"If you can't control your mind Something will happen It's just a question of time"

But she says she can only control it for so long:

"Oh babe I can not concentrate I'm here for the wrong reason

It's you who makes my knees shake One season, then The brakes're gonna break"

Somehting like that?

My Interpretation