I'm not sure what the recommended method is for learning to sing while playing, and I can only do it while playing chords really.

I learned by hitting each chord once only on the change whilst singing, and then building the strumming patterns around that. I've found that I can separate the vocal line from the guitar line in my ear, so the two don't confuse each other, but it took me a few months to get there.

Like everything else, it's practice and persevering.... :D