The localizer (component of the ILS responsible for lateral navigation) has a limited area of usability. In real life, it's something like this:
(Can't say if DCS has identical dimensions)
The glideslope extends to approximately ten miles and spans approximately 1.4 degrees elevation. Your task is to intercept the glideslope from below at an altitude of around 2,500 feet above ground level. If you'd like to approximate a real world ILS approach, simultaneously intercept the localizer and arrive at this altitude when you are about 10 miles from the airfield.
If you aren't sure how to read the F-15's HSI in FC3, I believe the manual does explain it in fair detail. If there's a catalog of ID numbers, it's there.
If you are looking for more information about approximating real-world instrument procedures, feel free to ask. Good luck.