SeaFoam

I've been getting misfire codes and poor mpg. I have already replaced plugs, wires, air filter, etc, with no difference. I would like to run some SeaFoam through the engine, as various other threads have suggested before trying the expensive repairs. What exactly is the procedure for cleaning the intake with SeaFoam? I have seen threads that say to pour it in the PCV valve area, and others that say to pull it in with a vacuum. What do you suggest? Detailed instructions or pics would be very helpful for either method. I have a 1996 Impreza 2.2.

Oh, can this really cause any damage to the car? I read somewhere that all the junk it blows out of the motor can gum up ur cat.

Thanks.