Choose a brush tool using Paint > [Brush type ] > [Brush name] and drag in a view.
See also Sketch on a Cintiq.
To select a brush and sketch with it
The Pencil, Marker, Airbrush, Pastel Soft, and Solid Brush tools let you drag the stylus to apply strokes of paint to a canvas plane.
The Eraser tools lets you drag the stylus to remove paint from a canvas plane in strokes.
When a brush tool is active, the cursor becomes a crosshair with the letter P (paint), E (erase), H (hide), or S (show).
When you hold the cursor over a canvas plane, the cursor also displays a preview image of how the brush will apply paint to the sketch.
Tips when using brushes
See Set up my tablet and stylus.
See Change your view of a canvas plane.
Notes when using brushes
The rest of the Alias windows are hidden while the brush is down. You can go to Preferences > General Preferences and change the setting for Hide Windows During Brush Stroke to change the amount of time (in seconds) that it will take before the windows reappear.