Screaming is tricky, because you can't physically stop them from doing it (like you can with hitting, throwing toys, etc).
I usually start with very little reaction, other than using a calm voice to remind them to use "inside voices". I also try to pick up on the triggers. Is it always in the high chair while waiting for food? I make sure he/she is the last one seated, or has an activity to stay busy. Is it while eating? Food gets removed when screaming occurs (usually it's because they are done), and another reminder about inside voices.
If it continues, it becomes a consequence-event. A time out, or whatever you choose to use.
I used to have 2 little boys who would shriek at each other, and laugh, then shriek, then laugh.... I turned them both around to face the wall/corner. Not so fun anymore!! LoL