IFR are detailed in ICAO Annex 2: Rules of the Air, Chapter 5: Instrument Flight Rules.
JAR-OPS 1.652 and associated guidance material specifies the flight and navigational instruments and associated equipment required for IFR or night operations. This may be supplemented by requirements contained in national Aeronautical Information Publications (AIPs). Minimum equipment lists (MELs) detail the conditions under which IFR flight may be commenced or continued when elements of aircraft equipment are unserviceable.
JAR-OPS 1.960 details qualification requirements for pilots to carry out IFR flights.