Think of subordinate information of information that is of secondary importance in the sentence(s).  So your first step is to determine a hierarchy of importance between the facts presented in the statements.  The part that is of least importance should be surrounded by commas. 

The most important thing about Skorsky is that he invented the helicopter because that is what made him famous; therefore, the information about where and when he was born is the subordinating information.  The sentences should be combined like this:

Igor Sikorsky, who in 1885 was born in Kiev, Russia, invented the helicopter.