One of the easiest ways to get to and from Modlin, Contbus is just a green-and-white bus that picks you up in the center of the city and takes you to the airport, and vice-versa. What's simpler than that! Plus, there's AC, Wifi and comfy seats -- but make sure to book ahead of time. Though these buses don't tend to fill up, it will make your life easier to book online. 

The trip will run you about 30 zł (far cheaper than many other options) and picks up either in front of Modlin (small airport so you'll have no trouble finding it) or on Nowogrodzka 62B (by the former Warsaw Mariott Hotel)