Obviously it was time to Make Something.
Step 1: Materials and Tools
(That was a really short step, wasn't it?)
Step 2: Design
If you can't think of your own idea, there is an excellent source of ideas is this database of plans HERE. They are all intended to be carved from plywood, but you could print one out and trace the design onto your card.
One important point to remember is to round the ends of the wings - it flies better, and it'll do less damage if you clip an innocent bystander.
A note for the sinister;
If you are left-handed, then you will need to make (and throw) your boomerangs in a mirror-image of what I show here.
Step 3: Cutting
Step 4: Taping
Then, wrap the whole thing in duct tape. Try and avoid lumps and flaps that will spoil the airflow around the wing.
Step 5: Flying Your Boomerang
The best weather for boomerangs is still air, or a light wind. Stand facing into the breeze.
Hold the boomerang vertically in your right hand, between finger and thumb, with the curved surface towards you (your thumb will be on the curved surface, and your forefinger will be on the smooth surface).
Lean the top of the boomerang slightly outwards, swing your hand back over your shoulder, and then flick it forwards rapidly.
It is vital that the boomerang is released with a powerful flick - if it doesn't spin fast enough, it will not return.
It may sound silly to say, but boomerangs don't fly in straight lines. When they fly properly, they fly in loops.
As you are facing into the wind, you are facing the furthest point your boomerang will fly. Throw your boomerang out to the right, about 20-40o from the way you are facing (if you are facing "12" on a clock-face, aim between one and two).
Hopefully, the boomerang will come back. As it flies, it will tilt, and be almost horizontal be the time it gets back to you. It will still be spinning quickly, though, so the safest way to catch the boomerang will be the clap it - smack two flat hands together to trap the boomerang as it passes.
Boomerangs are fun. They are cool. They let you show off when there's not enough wind for kiting. They can, though, be dangerous - spinning rapidly, moving quickly, they can cause potentially-serious wounds to the unsuspecting passer-by. Make sure your flying area is clear, or at least that everybody in it knows you are hurling unpredictable missiles around the place.
Throwing advice from Australia
Throwing advice from the USA
(There are also plenty of videos on YouTube. I have not made a "how to throw" video of my own because I am really bad at throwing boomerangs.)
But, above all, have fun.
(Caveat: I lifted this step whole-sale from this previous instructable)