How to wire up the XP Power 8-pin isolated DC-DC converter?
That series of DC-DC converters is available as both single output and dual (+ and -) output versions.

The IEU0312S12 has a single 12 volt output, so you use the "Single" column in the pin connection table.

You should apply the input voltage to pin 1 (negative) and pin 4 (positive).  Take the output from pin 5 (positive) and pin 7 (negative).

For the dual output versions, "Common" is the "zero volt" point between the + and - outputs.