@Pawel D. Very nicely explained. I'll just add that for LOC-only approaches, you wouldn't use APR, only "NAV". "NAV" is basically the roll mode, "APR" in addition also arms the GS pitch mode. So for localizer-only approaches, only use "NAV" and leave "APR" off.
Little trick for achieving a constant-angle descent on a non-precision approach:
Stabilize your airspeed.
De-select any active pitch mode (e.g. when in VS, push the "VS" key again to de-activate the mode). This reverts the aircraft into pitch-hold mode.
Use the UP<-->DN scroll wheel on the AP panel to manually adjust the pitch the autopilot will hold.
Fine tune the pitch until the green FPV marker sits over the desired glidepath angle.