Remove the old plugs, check every plug wire for visual damage using a multimeter check the resistance of every plug wire. In my case the resistance must be 2k ohm. Replace complete set if one is found to be out of specfications. Replace the plugs and new plug wires.
Now check the wiring harness suppling power to the coil packs test all wiring and replace or repair.Remember you cant just plug and play as the computer that drives that system is a fine tuned instruments. All the restiances and voltage must be equal or you will damage the CPU of the computer.