cardboard is ok as long as it is not treated with anything. A wooden table without any finish on it is also good. Best practice would be a grounded ESD mat with wrist-strap & at least 1 mega-ohm of resistance between the mat surface and the wrist strap, measured with an ohm meter. Remove your PC from this mat if you plan on applying AC power to the supply. Running energized gear on your grounded ESD mat while wearing your wrist-strap is asking for a lot of trouble.