Use fresh water from a kettle. Water left in the kettle loses vitality and makes tea taste dull. If you are using a pot, rinse it with boiling water to bring out the tea's flavour.
Add 3-4 teabags for a 6-cup pot, or 1 teabag for a mug. Pour in boiling water & leave to brew for 3-5 minutes. Add milk or lemon.
Whether you add milk before or after is open to debate.