Buy and Download >
Unzip the File. Copy the file to your Illustrator > Presets > Brushes folder.
Restart Illustrator. Load the brushes by clicking on “open brush library”.
Using complexity  Brushes
Draw a line or curve using paintbrush or pencil tool. the brush you will like to use and adjust stroke width
PS: you can expand the design later and scale it to any size without loosing the LOOK of your final outcome