step 1, get a factory service manual this will have the step-by-step procedures to how to diagnose/test/and ultimately repair the problems
step 2, get a real scanner so you can read data of what is causing the codes.
step 3, google search the codes to get info on what they mean. like I just checked for you and a p0420 is a catalyst failure. now you'd need to use testing procedures laid out in the FSM to determine the failure and how to correct. P0302 is a misfire on cyl. #2. so maybe you have a bad wire, bad valve, bad injector, bad rings, bad head gasket, or something like that. that's where a shop manual and scanner can pinpoint the exact problem