Get someone else to do it? Just ask a more experienced physio to do it and show you how. I don't think anyone is going to think less of you because of it. They may think you are an idiot if you try something you haven't done before on someone so sick!
Also, why 9L of oxygen through the nose? I thought that the max was 2L/min and anymore wasn't going to be effective?
Mind you - i am a musculoskeletal physio hahahaha