A kyusu is the best instrument for brewing sencha. But when it's not available, I would brew my sencha in this way with a mug, a saucer and a glass:
1. Put 3 gram of dry loose tea leaves in the mug (let tea leaves barely cover the inner bottom of the mug).
2. Pour in hot water of about 160F (I would boil the water and let the hot water pot sit on stove for 5 minutes).
3. Cover the mug with a saucer and let it sit for 45 second (adjust it depending on tea quality).
4. Use the saucer to cover the mug, and pour out all tea water in a glass and let the leaves stay in the mug (skills needed and some tea water may be spilled).
5. Enjoy the first infusion. Then brew for the second infusion in the mug for 20 second, with the mug covered by a saucer.
6. Pour out all tea water of the second infusion.
7. Brew for third infusion in the mug for 45 second, with mug covered by a saucer.
8. Do more infusions as wish.
17 hours ago