Its actually quite easy once you get used to it. There are many tutorials available to guide you though the process. There are a few programs that are freeware to help you. They can be very simple. I recomend a program that while not free is been just wonderfull its called "Flight Simulator Ad On Manager". It will actually make adding traffic SO easy. You have to have Lee swordlys Traffic Tools first. That is free. You can get both at www.flightsim.com.
Common sense is not all that common