Compare, do not contemplate
A single cup tells you very little, because you have nothing to measure it against. Two cups side by side tell you a great deal. Brew the same tea two ways with one variable changed — five degrees, thirty seconds, a gram — or brew two teas from the same category together. Almost everything anyone knows about tea flavour was learned this way rather than from a single attentive cup.
What to attend to, in order
Smell the dry leaf, then the wet leaf after the first infusion — the wet leaf is far more informative and almost nobody does it. Then in the cup: aroma first, then the initial taste, then how it feels in the mouth, then what remains after swallowing. That last one, the finish, is where quality shows most clearly and where cheap tea and good tea diverge most obviously.
Separate what you taste from what you feel
Bitterness is a taste. Astringency is a tactile sensation — a drying, puckering grip on the tongue and cheeks. Body is weight. Sweetness in tea is rarely sugar-like and more often an impression of roundness. Keeping these apart is the single most useful discipline in tasting, because they have different causes and different fixes, and running them together is what makes most tasting notes useless.
Write it down, badly
Notes do not need to be good. 'Smells like the inside of a wooden box' is a more useful note than 'complex and refined', because you will recognise it again. Record the tea, the parameters and three words. After twenty teas you will start seeing patterns you could not have been told, and your vocabulary will build itself out of your own comparisons rather than someone else’s.
What not to worry about
Slurping, spitting, formal cupping sets and scoring scales all belong to professional evaluation and are unnecessary for enjoying tea. So is agreeing with anyone else’s notes. If a tea reads as apricot to you and stone fruit to the catalogue, both are fine — the point of a note is to be useful to the person who wrote it.