What's your musical background? My original background was piano, and pianists get taught music reading and theory in a way it seems like vocalists and most other instrumentalists don't. Any piano book aimed at beginners (my favorite is the Alfred series) is going to have nicely stepped lessons about reading music. You can probably find pdfs of Alfred.

Of course, since you're teaching a vocalist and not an instrumentalist, there's the complication of ear training and vocal production at the same time. The choir directors I've had who have been the most effective at teaching reading have used Curwen/Kodály solfeggio and hand signs - there are a lot of aspects to that method that are effective at linking up what people subconsciously know about scales and intervals into an understanding they can use. I haven't brought out this method with my choir, but I may do so in the future. The most important part, as far as I can tell, is to keep an awareness of where the tonic is in the person's mind - a purely intervallic method can get people lost if they misjudge one interval.

Good luck with it! It's an intimidating task, and teaching any complex skill takes a lot of time and probably a lot of false starts.