The main question in most cases is whether to ruff before or after drawing the enemy trumps. In many cases drawing the enemy trumps first will deplete Dummy too of trumps, with none left at the end to ruff.
In such cases ruff first, then draw the enemy trumps.
In other cases, when you plan to ruff only once in Dummy, you can afford to draw one or two rounds of trumps first before executing the ruff.
When you have the luxury of lots of trumps in Dummy then it is usually best to draw trumps first before embarking on the ruff (s).

Example 1
Declarer is in a 4♠ contract.
North leads the King of Clubs which wins the trick. Then, seeing the singleton Diamond in Dummy, North cleverly leads a small trump at trick 2.

Declarer wins the trump lead in his hand, then plays the Ace of Diamond, followed by a small Diamond ruffed in Dummy.
Dummy leads a small Heart back to Declarer's Ace and Declarer then leads his last small Diamond, which is ruffed by Dummy's last remaining trump. Dummy then leads another Heart to Declarer's Queen, after which Declarer can finally draw the remaining enemy trumps.

Example 2
Declarer is in a 4♠ contract.
North leads the King of Clubs and wins. Seeing the abundance of trumps in Dummy (drawing them would be futile), North switches to a small Heart at trick 2. Declarer wins with the Ace.

In this case Declarer can combine drawing trumps with ruffs in Dummy.
At trick 3 he plays the Diamond Ace, followed by a small Diamond ruffed in Dummy. Dummy then leads the trump 8, overtaken with Declarer's Jack.

Declarer now leads his last remaining small Diamond for Dummy's second ruff (with the Spade 10). Dummy then leads his Spade Queen, which is overtaken by Declarer's Ace. This brings Declarer safely back on lead to draw the last enemy trump.

Example 3
Declarer is in a 4♠ contract.
North leads the King of Diamonds, which wins the trick as Declarer holds up his Ace. North then leads his trump Ace, followed by a small trump, won in Declarer's hand. .

Declarer, at trick 4, leads his Diamond Ace, then a small Diamond to Dummy's last remaining trump.

Declarer wins the second trump trick with the King, then draws the remaining enemy trump with his Spade Queen (discarding a small Heart in Dummy).

Declarer now leads a small Club to Dummy's Ace. Dummy wins, then plays the Club King, both Opponents following suit, followed by a small Club which is ruffed by Declarer. All enemy Clubs are now drawn.
Declarer now leads a small Heart to Dummy's King. Dummy then plays his three Club winners on which Declarer discard his last Diamond and two Hearts.

This example is similar as the previous one (Example 5), but there is one vital difference. Dummy has no side suit entry (the Heart King is now in Declarer's hand).
Declarer's Game is won or lost in the second trick. Dummy's Spade 9 can be the vital entry to Dummy's hand once the Club suit has been established. But only provided the Declarer keeps his Spade 4, his only trump card lower than the 9 !
Therefore on the second trick Declarer plays his Spade 10 under North's trump Ace, then wins North's trump lead at trick 3 with his King.

Declarer must draw the last enemy trump (otherwise they will ruff the 4th Club lead from Dummy), but first he must establish Dummy's Clubs.
Declarer, at trick 4, therefore leads a small Club to Dummy's Ace. Dummy then leads his Club King, followed by a small Club ruffed high by Declarer with the Jack (to prevent an over ruff and also to keep his trump 4!). All enemy Clubs are now drawn.

Declarer now leads his Spade 4 to Dummy's 9, which draws the last enemy trump and at the same time provides the vital entry to Dummy's established Club winners.

Example 7 (Not losing trump control)
Declarer is in a 1♦ contract.
North leads the King of Hearts, then the Ace, followed by the Heart Queen.

What do you do at trick 3 ?
If you ruff the trick you will have only three trumps left in your hand. If Opponents trumps break 4-2, which is most likely, you will lose trump control and never be able to set up winners in the Club suit. .

Do therefore a loser on loser play and discard you small Spade loser on the third Heart trick.

Now if Opponents continue with a 4th Heart lead you can ruff it in Dummy with the 10, and the 4 card trump holding in your own hand remains intact.
In essence what you have done is transfering your Spade loser to become a Heart loser instead.

Example 8 (Establishing an extra side suit winner)
Declarer is in a 4♠ contract.
North leads a small Heart. Dummy plays low and South wins with the King.
At trick 2 South leads another small Heart. What do you do ?

If you ruff the trick you gain nothing as North will then play low of course.
Instead discard a Club loser in your hand and let North win the trick with his Ace.

Dummy's Heart Queen has now become a winner. After drawing the enemy trumps, ending up in Dummy, you can lead the Heart Queen and discard the second Club loser in your hand.
You would always have made your contract, but this way you make 11 tricks instead of 10. A winning board in any duplicate competition.

Example 9 (Avoiding being over ruffed)
Declarer (West) is in a 4♥ contract, after North openend with a weak two bid of 2♠.
North leads the King of Spades followed by the Queen, his Partner signalling a doubleton (playing high - low first the 8 then the 2).
At trick 3 North leads his Spade Ace. What should you play in Dummy ?

With only small trumps in Dummy, South is sure to be able to over ruff.
Discard therefore a small Diamond in Dummy's hand, and let North win the trick with his Ace.
If he leads it once more, you can ruff it high in your own hand.