Take, for example, a birthday party for puppies attended also by babies. A catastrophizer should be able to look at a shining furry or hairless face and think: "Not for long will those faces be shining. Soon the vicissitudes of life will take their terrible toll."
But perhaps that's too obvious. Also, there are many people who will dislike puppy/baby parties simply because Society tells them they should find such things adorable. So the example really should involve something that is not overly German or overly puppy-ish.
Let's consider, then, a lazy Sunday afternoon doing chores and watching a bit of TV. It's not a great day, but neither is it a regrettable one. Let the catastrophizing begin. There are a number of possible jumping-off points:
This is a Sunday and it is not bad. Tomorrow is Monday and will probably be bad. Then there will be a Sunday again. And a Monday. Everything changes and ultimately comes to an end. Like life, and mine in particular.
This is not a great day. But later on in life, when I am aged and infirm, I will probably look back on such days with yearning because I will have no friends and no television.
How much of my life has been spent doing chores? Just keeping things going? Doing things that are not actively enjoyable? A great deal. Waiting in lines. Commuting. Going to pot-lucks. It's possible I will end up having had one really good hour of life by the time I die. Not that that will matter. I'll spend much longer being dead.
Remember: if catastrophizing does not come naturally to you, never fear. Or rather, always fear. And soon you'll forget you ever did anything else.