How to Use Lightroom 3 Brush

To use a lightroom brush, press the K key.
press the K key to activate a brush

Before you start brushing, adjust the levels to fine tune your brush.
adjust the levels to the right to fine tune your brush

You can hold down the Ctrl key as you paint to switch auto-mask on or off.
hold down the Ctrl key as you switch auto-mask on and off

You will see a pin where you started editing; you can adjust some of pin’s settings at the very bottom of the screen.
you can adjust the pins’ settings at the bottom of the screen

Press the H key if you don’t want to see it, but the pin is useful when you want to go back to an adjustment mask to edit later.
Press the H key to hide this feature

Press the O key to toggle the mask overlay, the Alt key to remove an adjustment mask, or for further color adjustments use the color picker.
color picker will let you make more fine tune adjustments