How do you install a gas pedal spring?

How do you install a gas pedal spring?
Usually the throttle return spring is built into the carburetor or throttle body, depending which your vehicle has. I'm going to suggest that this is not likely your problem. Double check that there is nothing underneath the dash holding the pedal (it's common for the under dash trim to fall against the arm attached to the pedal.) There could also be something jammed in the throttle body.

If after inspecting for foreign objects you still think you need a throttle return spring, it's a tricky job and it really depends on what vehicle you are working on. If you simply have a spring, then have someone move the throttle with the pedal and watch how it moves. Attach your spring so it holds it closed. Carburetors will likely have a good place to attach the spring. Throttle bodies (fuel injected cars) are meant to be replaced.