Air carrier Check Pilot reported failure to follow SOP resulted in crossing the final approach fix at MGGT one dot low. No terrain alert was received; and the aircraft landed safely.

Narrative

VFR day going into GUA. We thoroughly briefed all threats flying into GUA but forgot about not arming the approach until established on final. Autopilot was on; aircraft was gear down crossing AUR VOR at 9;000 ft. with LNAV and VNAV PATH armed with 6;300 ft. set in the MCP window. Once on teardrop continued to extend the flaps to 40. In the turn to final the glide slope came alive and we armed APP mode. The aircraft captured the false glide slope and made an abrupt descent to capture the glide slope. Captain turned auto pilot off and climbed back up to altitude; but we were one dot low crossing the FAF. We never received a terrain warning. We captured the glide slope and continued the approach into GUA. CA (Captain) touched down approximately 600 ft. down the runway. Lessons learned: stay in LNAV and VNAV PATH and set 5;100 ft. on the MCP; arm the approach on final. We considered a go around but the aircraft met the stabilized approach requirements. We armed the approach too soon with the auto pilot engaged. Stay in LNAV and VNAV PATH until established on final and then arm the LOC and then APP.