You can use the Link-OS Multiplatform SDK to facilitate your development for the QLn420.
It can be downloaded from the following link:
It contains sample code in C# as well as other languages such as Java and Objective C.
It does come with working examples as their corresponding source code.
There is no special setup needed on the QLn420 in order to get it to work with SDK other then configuring the printer for the proper network or Bluetooth connection.