Step 1: Materials
Thin card - eg. from cereal boxes
Glue - I use Evo Stick Impact glue, but a glue stick or UHU should work fine.
Newspaper - To stop glue from going on surfaces if using Evo Stick or UHU.
2l drinks bottle - Optional
Step 2: Template
It should measure 11.6cm along the bottom and 5.7cm from top to bottom
UPDATE: Also try drawing the template onto a 2l drinks bottle - it is ready curved and is transparent, very cool.