How to Make a Scatter Brush in Adobe Illustrator
You can create the brush using any vectorized shape(s) you create, you just select your shape (or shapes using the selection tool + the Shift key), & then drag the selection to the Brushes palette, a box would pop-up, choose "New Scatter Brush"; another box will pop-up to let you customize settings, just click OK for now, don't change anything.
Now test your brush, create a shape using the Pen tool or Pencil tool & apply the brush on it, wouldn't be perfect, let's fix it now:
Keep the shape selected, double click on the icon of your brush in the Brushes palette, the box to customize the brush will pop-up again, check the "Preview" box to see how the brush looks when you change settings & start changing them, when finished click on OK.
Here you are, your new & own scatter brush is ready!
You can also customize the brush settings only for some shape & not the others or the original brush itself, for that, select your shape then look for a button named "Options of selected object" in the bottom of the Brushes palette, click on it, brush settings box will pop-up, mess with it as you like before you save clicking on OK.