As soon as the dummy goes down you look at the suit you want to track, and say in your mind all the missing cards. That is all the cards you cannot see. Then as each previously unseen card is played you remove it from your mental list and this is the important bit - once again say the complete, now shorter, list to yourself. Let's practice -
And the missing hearts are? Say them to yourself.
King, Queen, nine, eight, four, or K Q 9 8 4 or however you best think of them.
Cornelia asks the class to quickly type the list out so it can be seen by all. Repetition is the key to learning. The class has to repeat the list to themselves and so must you.
So go on do it, as each previously unseen trump is played, repeat the now shorter list to yourself.
A key advantage of this method is that as the hand progresses you have less to remember. Beginners in particular should never again find themselves wondering 'has that 10 been played'.
Once you have mastered the art of tracking one suit, the next test comes when you also try to track a longer list as a defender and then graduate to tracking two suits at once.