Getting RTK-precision GPS with arduino devices like NavSpark?
How about the http://www.navspark.com.tw/tutorial-4 tutorial?  It seems to indicate you need a couple of the more capable $80 NS-RAW units with carrier-phase info rather than the less capable $22 NAV-SPARC plain GPS units.

The RTK system doesn't transmit only its positional error, (which it knows from external information) but also communicates the phase information for the particular channels, and then the RTK software re-calculates the positions of both using common sets of satellites and the phase info.  With the same framework, it knows the relative positions accurately, and with assumptions about the location of the base station, it can correct mobile station

This isn't an arduino problem, unless you are asking about porting RTKlib to something arduino.