Get a copy of the correct approach plate for the correct airport and
correct runway and print it out. If it's a FS mission, you can print
the plate out from the briefing area if they show one. Otherwise you
can get them free on the web at:
http://www.naco.faa.gov/index.asp?xml=naco/online/d_tpp

At the top of this chart (plate) note the ILS localizer frequency and
set it into your #1 Nav spot. This is the "localizer" frequency and
starts with the letter i in the identifier shown, like I-ORD for
Chicago ILS 14right runway.

Look at the lower part of the chart showing the profile. An ILS has a
sloping glide path to the airport. At the top of the path
approximately 4 to 5 miles from the airport, shows an initial altitude
to intercept. (some man not have this level section) You need to get
your aircraft at this altitude and be about 6 to 8 miles from the
airport pointed toward the airport at the final course angle as noted
on this chart. If it's an ILS 23, you'll be headed on a course of 230
degrees. That course should take you to the runway. You basically
keep the two crosshairs centered on the Nav1 display. If you do it
will take you right down to the touchdown zone of the airport.

I would recommend also loading this approach in the GPS, then it will
draw a line and you can see the point roughly 5 miles from the airport
where you need to intercept both the glide slope and the localizer.
The localizer will show as the vertical needle on the Nav1 and the
glideslope is either a horizontal needle or and little star that moves
up and down, center being the position you want to hold as you descend
down the glide slope.

Lastly, you could set the frequency in on the ground and take off
flying the approach backward trying to keep the two needles centered.
At 6 or 7 miles out, save the flight, then practice turning around and
flying it back in. The map mode in FS can also show you where you are
on the approach path. ... aKiley