You can draw in the sprite editor using tools
like the Pencil 
(B key), you can use Left click to paint with the
Foreground color, or Right click to paint with the
Background color. This is true for almost all painting tools.
The tool behavior is also modified by the active ink.
The following is the list of available painting tools:
Pencil (B key)
Line (L key)
Curve (Shift+L key)
Rectangle (U key)
Ellipse (Shift+U key)
Countour (D key)
Polygon (Shift+D key)
Other tools that will help you in the drawing process:
Eraser (E key)
Eyedropper (Alt or I key)
Rectangular Marquee (M key)
Move Cel (Ctrl or ⌘ or V key)
Slice (Shift+C key)
Zoom (Z key)
Some helpers for the drawing process:
SEE ALSO
Sprite Editor | Ink | Tool Bar | Move Tool | Zoom Tool