Here's where your leading and stripe measurements come into play.
Cut a strip of your plain white (or coloured copier - who am I to say - no one will see this) paper about 8cm wide along the narrow side of the paper.
Fold this strip in half and add a strip of duct tape to the back of it - this will reinforce it so it won't rip while you're drawing your rainbow template(s)
With a ruler, draw a line down the centre of the non-taped side
Starting at least 1cm in from the edge on this line, make a small hole with your pin - that's your anchor point. If you're using a conventional non-mechanical pencil, you may need to make slightly bigger holes.
Add your next hole 1-2cm along from that first one - that's the inner edge of your first stripe
Now you're marking the width of that first stripe, so make your next hole 1 stripe's width along from that point, if you're following my measurements that'll be 2cm along.
The next spot is the edge of the first leading stripe - so make your next hole that measurement's distance from the last (for me - that's 1cm along)
Repeat those two steps, marking stripe width and leading width until you've made the hole for the far edge of the last stripe.