I had that problem, Blackcatter. Then i got the chords for Goodnight Irene and it all fell into place...don't know why!

Try strumming the chords in sequence until you feel comfortable with them, and then just join in when you feel ready. It doesn't matter if it doesn't sound right to begin with, just try it, then practise until you get it right...worked for me anyway! Try it with a song you know really well, both vocals and guitar.

Hope this helps:0)