2003 Volkswagen GTI have the codes, now what?

2003 gti 20 ae, like the person in aug, I had stumble when accelerating. Intermittent but continued all the way home. Read codes now how best to proceed. Codes po300 and po304 -multiple and cyl 4 misfire. So what pathway do I follow to track down the source of the misfire? Is it related to detonation? Dealer only items? I bought the car from the dealeer with 1100 miles on it and first time a mechanical problem (except for the windows falling out of the tracks and a seatbelt switch which is recurring) does the online book you promote have the answers in a simple format? Thanks rob.