There's not much to it, but a couple of small things make a difference.
1. Boil your water - or just heat it. The coffee will even dissolve in cold water!
Bring water to a boil and let it sit for 30–60 seconds. Instant coffee can taste flat if the water is too hot — just off the boil is the right temperature.
2. Add coffee to your cup
Empty the sachet (or measure out 2–3g) into your cup.
3. Add a small amount of water first
Pour a tablespoon or so of hot water onto the powder and stir briefly. This dissolves the coffee properly before you add the rest of the water — it prevents clumping and improves flavour.
4. Top up and stir
Add the remaining hot water to your desired strength (200–250ml). Give it a thorough stir. That's it.
5. Finish it up
Instant coffee works well with milk, cream or non-dairy alternatives. Add it after the coffee is fully dissolved.