When you get a notification on your watch, spend 5 seconds thinking "Do I want this notification on my watch?" This question is apparently the easy part.
Now for the "hard" part: swipe to the left on the notification, tap the "..." button, and tap "Turn Off on Apple Watch". Now you won't get notifications from that app on your watch. Should take you about 2 seconds.
Now for the "hard" part: swipe to the left on the notification, tap the "..." button, and tap "Turn Off on Apple Watch". Now you won't get notifications from that app on your watch. Should take you about 2 seconds.