You're going to make mistakes—everyone does. Here are the ones I see most often:

• Using boiling water: This is the #1 killer of matcha flavor. It turns bitter and harsh. Always cool your water.
• Skipping the sift: Lumps are the enemy. Sifting takes five seconds and makes a huge difference.
• Whisking too hard or too little: You want a gentle zigzag, not a frenzy. If you whisk too hard, you'll get big bubbles that pop; too soft, and you'll have clumps.
• Storing in the fridge without sealing: If you do use the fridge, make sure the container is absolutely airtight. Otherwise, your matcha will absorb odors and moisture.