This is a tricky skill to get right, but once you have practised - a lot!! You will find yourself getting quicker and neater.
You will need Perga cutters or Parchment Scissors, parchment paper and a twin needle tool (this one is Bold). All instructions are for the right handed.
Perforate with the twin needle tool, neat evenly spaced holes. Place the very tips of the points of the cutters/ scissors into the first two holes.
Lower the cutters / scissors so that they are almost parallel to the parchment and keeping the cutters pointing straight (at a right angle to the perforations) twist slightly to the left (anti-clockwise) and snip.
Always move the parchment around to make the snipping easier. You should end up with neat little V shaped teeth along the edge. Hope this is helpful - just keep practising.