Focus on 1 chord at a time. Play through the song only strumming the C chords. Next time, only F.
Play through the song and pick 2 chords to strum. Practice switching between these chords 10 times.
Listen to each string at a time. If a string isn't sounding properly, it could be that you're not pressing hard enough, or that something is touching the string and it can't vibrate. Try to fix the problem then try again.
SLOW IT DOWN. This is such a clever trick.
Click the little gear at the bottom right of the YouTube video.