Tea boiled with milk, sugar and spices is not complicated. It's also easy to make badly, and the difference between good and mediocre comes down to a handful of decisions.
The tea
The most consequential choice and the one most often got wrong outside the subcontinent.
What's needed is a strong, malty black tea, typically CTC — crush, tear, curl — processing, which produces small granular particles that release colour and strength rapidly.
Delicate orthodox leaf teas, whatever their quality as a drink on their own, disappear entirely under milk and spices. Using an expensive first flush for chai is a waste of the tea and produces weak chai.
Assam CTC is the standard for good reason: high strength, malty character, and it stands up to boiling.
The order of operations
Where most of the technique sits, and where methods genuinely differ.
The common approach: water and spices boiled first, then tea added and boiled, then milk added and brought back to a boil, then sugar.
The reasoning behind boiling spices in water first is extraction. Water at a rolling boil extracts more from whole spices than milk does, partly because milk fat and proteins bind some compounds.
Adding tea to water rather than to milk also extracts more strongly, for similar reasons.
Adding milk last and bringing back to a boil integrates it without prolonged boiling, which changes the flavour of milk in ways that aren't desirable.
Methods that put everything in cold together and heat it produce a milder, less extracted result. Not wrong, and a different drink.
The milk ratio
Varies enormously by region and preference, from roughly equal parts water and milk to almost entirely milk.
More milk gives a richer, sweeter, milder result. More water gives a stronger, more astringent one where the tea and spices are more prominent.
Full-fat milk makes a substantial difference to body. Skimmed produces something noticeably thinner.
Spices
Enormous regional and household variation, and a few principles hold.
Whole rather than ground. Ground spices produce a cloudy, gritty result and lose their aroma quickly in storage. Whole spices crushed just before use are considerably better.
Crush, don't pulverise. A rough crush in a mortar releases oils and lets the pieces be strained out. Powder cannot be strained.
Cardamom is the foundation in most versions. Green cardamom, crushed pods included.
Ginger is added fresh, grated or crushed. Dried ground ginger is a different and harsher flavour.
Beyond those: cinnamon, cloves, black pepper, fennel, star anise, bay leaf, nutmeg. Regional preferences vary and household versions vary more.
The most common error is too many spices, which produces something muddled. Two or three, in adequate quantity, beats seven in small amounts.
Timing the boil
The variable that most affects the final character.
Longer boiling after the milk is added produces a darker, stronger, slightly caramelised result — the character of tea from a roadside stall where the pot has been simmering.
Shorter boiling produces something lighter and fresher.
Over-boiling makes the tea bitter and the milk taste cooked. There's a window and it's a few minutes wide.
The traditional technique of raising the pot to let the chai rise and fall repeatedly serves to aerate and mix while preventing boil-over, and it does affect the texture.
Sugar
Added during boiling rather than at the end in most methods, on the reasoning that it integrates rather than sitting as sweetness on top.
Quantities in traditional preparation are higher than most people outside the region expect. Chai is a sweet drink and reducing sugar substantially changes its character rather than simply making it less sweet — sugar balances the astringency of strongly brewed tea.
Common problems
Weak and pale. Not enough tea, wrong type of tea, or not boiled long enough before milk was added.
Bitter. Over-boiled after the tea went in, or too much tea for the liquid.
Tastes of cooked milk. Boiled too long after the milk was added.
Thin. Too much water relative to milk, or low-fat milk.
Muddled spice. Too many spices, or ground rather than whole.
Grainy. Ground spices not strained out.
The straining
A fine strainer, and pour from a height so the chai aerates on the way into the cup. That produces a slight foam and improves the texture.
Serve immediately. Chai that sits develops a skin and the balance shifts as it cools. It's a drink made and drunk, not made and held, which is why it's made in small quantities several times a day rather than in a pot for later.
Scaling and holding
Two practical points for anyone making it for more than one or two people.
Scaling up is not linear. A larger volume takes longer to come to the boil and spends longer at temperature, which over-extracts the tea. Reducing the boiling time when making a larger batch compensates.
And chai does not hold well. Kept warm, it continues extracting and the milk continues cooking, so a pot made half an hour before serving tastes noticeably different from one made at the moment.
Where it must be made in advance — for a gathering, say — the workable approach is to prepare the spiced water and tea concentrate ahead, and add the milk and bring to the boil in batches as needed. That preserves most of what matters.