Painting needs paint brushes and Blender provides a Brush panel within the Toolbar when in Weight Paint Mode.
In the Data-Block menu you find predefined Brush presets. And you can create your own custom presets as needed.
- Radius F
The radius defines the area of influence of the brush.
This is the amount of paint to be applied per brush stroke.
- Use Falloff
When enabled, use Strength falloff for the brush. Brush Strength decays with the distance from the center of the brush.
- Weight Ctrl-F
The weight (visualized as a color) to be used by the brush.
Using Ctrl-RMB you can set the weight to the value thats under the cursor.
- Direction D
Brush direction toggle, Add adds weight value while Subtract removes weight value. This setting can be toggled with D.