How to make use of Dummy in No Trumps:

You have found yourself in No Trumps with very little points in Dummy and a long suit.

The first thing to think about in No Trumps is how to make the tricks required (9 in 3NT).  Then think about how to make the most of the dummy provided.

In this exampe, there is a 6 card suit in Dummy.  It is almost always a good plan to make the most of Dummy's long suit.

After the lead (which you win in this example), play the K♦.  If the Ace takes it, win trick 3 and then play the Q♦ - if they duck the first ♦, still play Q♦ on trick 3.

Once the Ace is out, you now have a low ♦ to get over to Dummy.  Dummy has now provided you with 5 tricks.  Count the discards and you can see the 4th ♥ in hand is good and so throw the losers in ♠ and you are only left with winners in hand - 12 tricks made.

Key here is not to panic, don't be afraid to throw your high cards under the oppositions Aces if you need to keep a low card to transport to dummy.