When we talk about "chai-flavored" anything, in my opinion the dominant flavor we're really talking about is green cardamom. After green cardamom the taste of chai can diverge in a few different directions. However, these spices are perhaps most commonly used, and they form the base of the chai (as taught to me by my friend Baljit).
Primary chai spices:
Green cardamom pods - 7 pods - the key ingredient - give chai it's flavor - break pods in a spice pestle
Cinnamon stick - For sweetness and warmth - about a 2" stick
Fennel seed - Offers a gentle anise or licorice - about 1/2 tsp
Fresh ginger - Flavors the whole pot of tea with a fresher spice - a chunk about the size of 1/2 your thumb - from tip to first joint - sliced.
By personal preference:
Black peppercorns - Gives the chai a spicy bite - just one or two corns
Whole cloves - Musky and strong - try one or two at most, as it can overtake the flavor in chai
Coriander seeds - Sweet and mild, with a citrus note - just a few if you like coriander
Star anise - A darker, sweeter licorice flavor than the greener fennel seed - a few petals broke from the pod
Estimate how many cups of chai you want to make. The ingredients outlined above add flavor for about 4 - 5 regular cups of chai.
Measure out water for the number of cups you want, bring water to a boil on the stove. Add a bit more water as it boils on the stove for about an hour and reduces in volume. Add only the spices to the water (tea is added later).
Bring water/spices to boil then turn down to a low simmer.
When the water has changed color and you can see waves in the water from the steeping spices (about 20 minutes) add Red Rose tea bags (1 bag per cup - Red Rose is the brand preferred by East Indian chai makers).
Continue to boil the tea in the pot with spices until the water turns a deep orange/red/brown color (another 20 minutes). You should not see the bottom of the pot when it's the right color.
Reduce the chai from boiling to a very low simmer. Add milk (2% is creamy, whole is very creamy and rich). Add milk based on your own preference (like adding milk to coffee). Keep the chai on