Tip 1 -   Contract 4 ª by South           (Degree of difficulty = 10)

West leads the King of diamonds. How should South plan his play?

You have to estabilish clubs at dummy without letting East (the dangerous side for his retourn hearts) take a trick.
Since after the initial lead the Queen of diamonds is located in West, you should go to dummy with a little trump to the 8 and play the Jack of diamonds and discard a club from your hand instead of ruffing: you do not lose any trick, jou are just swapping a loser club with a loser diamond. Now, taken any retourn, you can  take another round of trump, play Ace, King of clubs and ruff a club, estabilishing the suit. You then go to dummy with the last trump and play the last 2 clubs on which you will discard 2 loser hearts.

Tip 2 - Contract 4 © by South       (Degree of difficulty = 12)

West leads the King of diamonds. How should South plan his play?

The true problem of this hand is not losing a trick at club, but not losing more than 2 tricks at spades!
You have to draw trumps, and then play the Ace of clubs. If the king doesn't  fall, you give him the trick, completing the elimination game, and you will wait op's retourn: you would not lose more than 2 spades!
On the contrary, if you take a finesse club, West take and switches spades: East would take and return with a club:
Now you would lose other 2 tricks in the spades suit and the contract would be set!

Tip 3 -   Contract 6 © by South           (Degree of difficulty = 12)

West leads the J of clubs. How should South plan his play? The game isn't sure, but you must try to increase probability!

After ducking the initial lead, you have promoted the queen which is now a winning card and on the Ace of diamond you'll discard a loser spade. Now you can try the odds at diamonds: if the split is 3-3 (67%) you have done your contract.
But you could increase the probability of about 5% if you draw only 2 rounds of trumps, then you play Ace, King and Queen of diamonds:
- If the suit is split 3-3, you draw the last trump and play the fourth diamond.
- If the suit is split 4-2,  and you have not been ruffed, you ruff at dummy the last diamond, you come back with a ruff at spades and draw the last trump (Manouvre of Guillemard).

In both cases you have made the contract; in the case your third diamond is ruffed, the contract is set but you have not lost any trick since your fourth diamond would not be estabilished and you now can ruff it at dummy with his last trump!

Tip 4 - Contract 4 © by South (Degree of difficulty = 10)

West leads the Queen of diamonds. How should South plan his play?

Timing, that's all! Your planning should foresee to draw trumps, to ruff or estabilish a diamond, to give a loser diamond and a spade. But what should the timing be?
Since you should give a trump (hoping to find the 3-2), it would be nice if you could utilize the third trump at dummy to ruff a diamonds, but it you take at once, when you'll give the third diamond, West would play trump and would take away the last trump at dummy.
So you have to duck the initial lead, take the second round, play trump twice and then play your last diamond you can ruff at dummy. Now you give the Ace of spade and the last trump and make your contract.

Tip 5 - Contract 4 ª by South                 (Degree of difficulty = 4)
West leads the 4 of clubs. How should South plan his play?

South should allow East to win the first trick. As long as West cannot regain the lead, the K of hearts is protected and the diamonds finesse can be taken in safety: South wins the club continuatio, draws trumps and takes the diamond finesse and, although it doesn't work, he makes 5 spades, four diamonds and one club.
In this hand, South has to ensure that West (the dangerous side for leading a heart) cannot take a trick again.

Tip 6 -   Contract 6 § by South           (Degree of difficulty = 5)

West leads the Ten of diamonds for the Jack of dummy and the Queen of East who then plays the Ace in the suit. How should South plan his play?

South ruffs the diamond return, enters dummy with the Ace of hearts and plays the Queen of clubs, running it if it is not covered by the King. If East plays the King, South takes with the Ace and, when West do not answer in the suit, he goes to dummy and plays a trump finessing the Ten.
The contract would be set if South, after entering dummy with the Ace of hearts, would have played little trump to the Jack: he would have lost a trick of trump!