Old school way is disconnect one spark plug at a time. When you have the offending cylinder disconnected the engine will sound no different but if you have one disconnected that is still ok the engine will sound terrible. Safest way to do it is disconnect one and start the car. Then switch off reconnect spark plug and move onto next.
change the spark plugs and see how you go. Failing that I do think it's going to be a garage job. Unless someone else can suggest something. Sorry I can't be more help than that.