ive never made one but there are a few tricks to do so that you can modify your o2 so they read clean and cool air
One of them is easily going to your local auto parts store and buying SPARK PLUG NON_FOULERS. you take the first and drill it out to 1/2 inch and you put it into the none modified on and then screw that into the o2 sensor bung. its all writen up here.
Im not sure if your trying to stop codes from popping up but its probably the cheapest and easiest way to modify your CAT or exhaust and stopping them