If I recall correctly, I had the the altitude set for 4000 ft as the RNAV (RNP) Y approach chart indicates for TEVUC. After passing TEVUC, I probably was too slow to reduce the constraint to 2900 ft. Well before JISOP, I reduced the selected altitude to 2900 ft as indicated by the approach chart as the glideslope intercept altitude. I immediately pressed VS and set a high descent rate to try to get down to 2900 ft before JISOP. Shortly after pressing VS, I also pressed APPR to arm the glideslope capture, which never happened since the actual altitude never got below the red glideslope dot. So, my actions probably prevented the glideslope capture from happening. However, I wasn't actually asking if I did the correct thing -- only what the correct procedure actually is.
So, if I understand What Pils said above, the procedure for an RNAV (RNP) approach is:
Fly this procedure with FMS1 or FMS2 selected as the navigation source.
Before TOD, select altitude setting to glideslope intercept altitude or altitude assigned by ATC, since this acts as a constraint on descent,
Press VNAV to engage VNAV mode
Optionally, press VS and set a descent rate. VNAV mode will respect the altitude constraints (and optionally speed settings) in the flight plan. Probably not needed.
Before the glideslope intercept, press APPR to arm VGP, the synthetic (computed) glideslope or glidepath.
When the glideslope is intercepted, VGP will activate and the aircraft will descend along that path. Once VGP activates, the altitude constraint indicated by the selected altitude will be ignored.
Cheers and thanks for the feedback. I will try this procedure now and see how well it works.