A reminder so you don't look as confused as I did.
Catcher's interference (usually, the batter hits the catcher's mitt on a swing) is hard enough to pick out from the press box. Why is the batter going to first on a ball that was fouled back into the stands? What did I miss?
Once you've got that under control, don't forget that you automatically charge an error to the catcher. (Once you figure that out, its no problem to not charge an earned run if that guy scores.)