How do you create the effect of a free form line drawing itself?

I can’t seem to google an appropiate tutorial for this so I thought I’d ask here…

I do know how to use the pencil tool and how to make objects follow motion guides…

I’d like to create the effect of the free form line that draws itself…My goal is an free form line that draws the profile of a person as a border, before I fade in the picture of the person…

If you know how to do this or can point me in the right direction please let me know…Thanks! :bandit: