When I started playing this song I loved the way the anchors worked, as the chords cycled around.
If you play the Big G, then leave your ring finger where it is as you change to the D, the leave your index finger planted when switching to the A, and then slide it back one for the E, as per Stage 1, then you can get a really good bouncy feel going. And the riff on the A and open D string adds alot of fun, if you keep your rhythm going too.
Maybe try two downstrums per chord at first, before adding in the upstrums.
That said, I have always had trouble singing it as I play.
Hope that helps.