I’ve been looking into implementing a custom online subsystem and AFAICT it doesn’t look too complicated, check the Steam implementation or maybe the Null implementation. Just implement all the relevant interfaces/abstracts starting from [FONT=Courier New]FOnlineSubsystemImpl. The code seems pretty self explanatory and with both the Null and Steam implementation, combined with good comments in the interface headers, you should be able to just do it. 
I can give you a better answer in a couple of months when I’ve attempted it.