As declarer, you should count your winners — or losers — when the dummy appears. Here, it’s easier to count losers. In 2♠, you have six possible losers: the ♠A, the ♥A, three clubs and (eventually) one diamond. To make your contract (which requires eight tricks, meaning you can allow the opponents to take five tricks only) you have to eliminate one of your losers. How can you do that?
First, let’s consider what you shouldn’t do. If you reflexively attempt to draw trumps, West will win the trump ace and continue to attack diamonds. When you win the ♦K and finish drawing trumps, you’ll have to drive out the ♥A, but when you do, the opponents will win and cash a diamond trick. They will then cash their club winners (if they haven’t done so already) and beat you one trick.
You can’t do anything about the ♠A, the ♥A or the three club losers: The opponents are going to get those tricks no matter what you do, but you can do something about that eventual diamond loser. The key is to find a way to discard it before diamonds get attacked a second time.
After winning the ♦K, play the ♥K. If the opponents win and play another diamond, win the &hdiams;A and cash the ♥Q and the ♥J, on which you can discard your diamond loser. Only then should you start trumps.