Two things. First, go buy and read Guitar Zero. You'll enjoy it.

To your question. Metronome and slow down the BPM. And, one thing you can do is mute the strings with your fret hand, don't worry at all about the chord.

Now, just strum (slowly) using the strum pattern you want. Like, Down-Down-Up-Up-Down.

Don't worry about changing chords, or speed, or even the sound of the chord at all.