Use cooler water than you would for black tea. A good starting point is about 1 teaspoon of sencha per 6-8 oz water, brewed around 155-170°F. Steep the first cup for 45-90 seconds, then make a quick second steep around 15-30 seconds.
If the cup tastes harsh, lower the water temperature or shorten the steep. If it tastes thin, use a little more leaf rather than much hotter water. Sencha is especially responsive to brewing, so small changes in temperature and time can move the cup from brisk and grassy to soft, sweet, and savory.