Air carrier Relief First Officer reported landing safely after the flight descended well below the glideslope on the approach to WSSS/SIN airport.

Narrative

We were at 5000' and on vectors to the ILS 20R into Singapore. The Captain was the pilot flying. The pilot monitoring was the FO; a probationary pilot on a consolidation flight. The Controller said to descend to 2500 speed 180. The FMAs read HDG SEL and FLCH. Then the Controller said; 'descend to 2500'; (although we were already in the descent to 2500) cleared for the ILS 20R'. We were on an intercept heading for the localizer and FLC descent to 2500 and the Captain armed the approach. The PM FO asked if he wanted 1500 (FAF ALT) put into the MCP. Both the IRO Captain (flying IRO) and I (in second IRO seat) looked at our approach and saw that as the FAF altitude however we were not yet established on the approach; so we recommended that 2500 be reset into the Mode Control Panel (MCP). We had already descended below 2500 and still in FLC when we reset the altitude.The PF then decided to kick the autopilot off and fly manually. He was still in a slight descent causing his speed to increase. The Controller quickly came on to tell us to reduce airspeed to 180 and then 150 knots. The PF called for flaps 20 and the gear and then flaps 25 prior to the gear coming down. The horn sounded until flaps 25 was met but it seemed as if the PF was clicking his A/P disconnect on the yoke because the horn kept going off. At this point I stretched my neck around the PM's right shoulder; and I noticed we were more than 2 dots below the glideslope and still descending. Again; we were still in FLC with 2500 set in the MCP but we were now close to 1000 ft. I called out we were way too low. The PF slowed his rate of descent; but we were still descending. Again; I called out way below glideslope. He said correcting. The GPWS 'glideslope' warning then came on several times and each time the PF said 'correcting'. I was planning on calling go around at 500' but the PF leveled at approximately 700' and drove into the glideslope at 700'. By 500' we were stable and continued to an uneventful landing. We debriefed what happened thoroughly.