A hera marker is a small tool with a sharp curved edge. It is designed to fit in your hand easily (even if you’re left-handed!).
Hold in your hand with your pointing finger braced on the top for control. Press onto fabric with a back and forth motion (like a saw). It will leave a clear crease on the fabric.
This is ideal because it does not add anything chemical to the fabric. Sometimes pens, pencils or chalk will leave permanent marks. A hera will solve that problem.
It can be used on both straight and curved lines.