Whilst you can do this with tracing paper or cardboard, you will find it much better with some thin transparent plastic you can cut easily with scissors. Look out for suitable flat pieces on packaging - plastic milk cartons come highly recommended!
1. Carefully trace around the closed outline onto your plastic, mark on the position of the spring and pins:
2. Cut it out and cut the spring portion away from the rest.
3. Put pins through the spring to hold it in place on the drawing, put the remaining piece in position, spike a point through the pivot and turn it out into the open position.
4. Draw the blade shape you require on this piece, remembering any alterations you make to the back of the blade will alter how it looks closed, so keep turning it into the handle to check - do not alter the part of the blade inside the handle in the open position.
5. Cut around 5mm from the length of the spring; it needs to be longer than the pivot point but not go as far as the end of the knife.
6. Put the spring back on the drawing, put the blade in the open position on top, then carefully scribe onto it where it will meet the spring (back square) and cut this portion from the blade.
7. Turn the blade into the handle again, put another piece of thin plastic over the top and sketch off the handle shape, making any necessary adjustments to cover the back square or to expose more of the blade to make it easier to open - carefully mark on the position of the pins - cut it out.
Here you can see knife deigner Jeff Durber making up a template for his knife:
