Does the vehicle run rough? Does the check engine light flash while the car is running? There are many "Possible Causes" of a P0300 Random Misfire code. I would reccommend taking it to a shop if you don't have access to some basic test equipment such as a scan tool (not a code scanner), DVOM, Fuel Pressure Guage, Ignition Analyzer, Injector Tester, etc. How did the old plugs look when you removed them? Fouling? Deposits? Wet? If they were fouled or had deposits, what color was the debris?
I agree that looking for a vacuum leak is the first step. Using carb cleaner, spray along the intake and throttle body areas. If you don't hear one or the engine RPM does not fluxuate when you hit a certain spot, you probably don't have a leak. Make sure your wires are on the right plugs as well. if the wires are right and you don't find a vacuum leak, then I would reccommend taking it to a shop for proper diag with a scan tool and fuel injection test equipment. Unless the coating on the spark plugs was thick like chocolate syrup, then you have nothing to worry about. A light brown coating is normal given all the fuel additives in gasoline these days.
Check for a vacuum leak. Get a carb. cleaner and while vehicle is running, spray the intake hoses and listen for engine surges. If you spray and hear one, you have found your leak and replace that line.