you intercept the localizer at a given altitude, the maintain that altitude until the glideslope "comes alive". Then, you remain on the localizer for left and right guidance while the glideslope tells you if you are high or low.
Both are essentially the same thing, just aligned at 90 degrees to each other. The localizer (LLZ) will tell you if you are left or right of the extended runway centreline* while the glideslope (GS
) will tell you if you are above or below a nominal glideslope.
*There are localizers which are not aligned with the runway, offset localizers. There are even complete LLZ/GS
combinations which lead you to an off-airport spot, intended to get you safely to a point where you can continue the approach using other means. In those cases, they are not considered ILS systems though.
I thought I was doing good trying to avoid those airport hotels... and look at me now.