you dont use the piece to make the template...detach..use a clear shower cap...the following is copy and pasted
put the cap on make sure it was pulled down all the way and tight to my head. use some scotch tape to go around my head so it wouldn't move around. I taped just above my eyes so I have plenty of room to work with when tracing my head shape onto it. Now that it is secure to my head I trim the excess below off, which is a lot. It covers almost my whole face when it's pulled all the way down and tight.

At this point I'm comfortable with it to draw on. then apply a few layers of packing tape..front to back...side to side and then again front to back. this stiffens the template and retains the shape.

Once this is done I take it off, and cut according to the shape you drew.
of course then you would want to re attach.
i would say an inch or so. if your hair has varying colors then take a few samples... from different areas, put them in separate bags and mark where each came from.