The diagram depicts two springs, look closely and you'll see a second spring (#13) closer to the accelerator pedal. That's the one you need and where it should be placed. There's a small opening - sometimes there's a small tab - in the toeboard where the lower end of the spring hooks - use a spring with sufficient tension to pull the pedal back when your foot is removed the from the pedal, but not so stiff that it's hard to push the pedal down.