Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human-Machine Interface; Training / Qualification; Situational Awareness
Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Environment - Non Weather Related; Human Factors
While on the ZZZZZ4 [arrival]; approaching ZZZZZ we were given a vector to the North.  We were asked if we had the river in sight; which we responded; 'Yes.'  We were cleared the ZZZZZ visual to Runway XX.  CA wanted to cross ZZZZZ1 at 3;000 feet; so CA selected Flt Lvl Chg and proceeded down.  Upon reaching the fix; CA failed to re-select VNAV as we commenced the approach.  LNAV was selected and we were tracking correctly.  Between ZZZZZ2 and ZZZZZ3; CA realized we were too low.  CA saw the '1;700' at ZZZZZ3; we were at approximately 1;100 feet.  CA clicked off the auto pilot and leveled the aircraft at 1;000 feet.  We were given a 'low Altitude' alert from the Tower.  We maintained our altitude and tracked the approach.  We landed without any further incident.  We were below the charted altitudes at ZZZZZ3 and ZZZZZ4; possibly ZZZZZ5.I believe currency was a significant factor.  The CA had a total of 55 hours in the preceding 5+ months and the FO had just come back from 6 month leave.  Because this was an RNAV approach; briefing the importance of verifying both LNAV/VNAV on the FMA would have reminded both pilots to look for this upon the commencement of the approach.
We were descending for the ZZZZZ Visual approach to [Runway] XX. We had the RNV-F approach selected in the FMS. Descending through approximately 5;000 feet ATC had given us a lower altitude so the Captain selected LVL CHG. A short time later ATC asked if we had the river in sight; we did; so we were cleared for the visual approach. We set our briefed minimums for the approach and continued our descent. However we had not gone back to VNAV and confirmed VNAV PATH. As we approached 1;000 feet we both realized we were too low. The Captain disengaged the autopilot. Stopped the descent until we were reestablished on the path. From there we continued the approach to landing.This was my first leg back from a six month leave and the captain is on reserve and had not flown in two months. We had briefed before the flight that our biggest threat was our lack of recent flying. This proves that even when a threat is identified and discussed; it does not go away.The company and [union] need to continue to stress as more of us return to work that observance to SOPs is vital to the priority of safety.
Air carrier flight crew reported descending below charted altitudes while on approach and cited lack of recent flying as a contributing factor.