Just open the Illustrator file (it works in Illustrator CC only) then open your Brushes Window.
Draw any path and apply one of two marker brush designs. Increase or decrease the stroke weight to change the thickness of your marker line.
Marker Brush for Illustrator CC is a free download for TheVectorLab Newsletter subscribers, and is also included in Graphic Design Launch Kit.Go here to sign up and get all the free downloads.