Falcon 2000
Flight Crew Instrument; Flight Crew Flight Instructor; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Training / Qualification; Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Human Factors
We were flying to ZZZ; assigned the XX LDA starting at ZZZ1 VOR. We were instructed to cross ZZZ1 VOR at or above 15;000 and cleared for the approach. We had programmed the avionics to fly the approach and descend on the approach to the glide slope intercept altitude. During the setup we neglected to arm the VNAV. After passing ZZZ1; descending; the aircraft did not level off at 14;000. We noticed the missed level off just before the controller alerted us to a probable altitude deviation. The Captain immediately disconnected the autopilot and returned to the appropriate altitude. The flight concluded uneventfully.I believe there were several factors that contributed to the event. Primarily; the flight crew was legally current but a little out of practice.  A full procedure instrument approach in real-time in mountainous terrain is different than currency and maintenance flights in non mountainous terrain. We have a very strong safety culture; attend recurrent training more frequently than required; and have extensive backgrounds including airline and corporate jet experience but are still susceptible to being 'rusty'.
[Report narrative contained no additional information]
First Officer reported flight missed an altitude level off because they were a little rusty.