This site is being deprecated.
Please see the official X‑Plane Support page for help.
You might want to try looking it up online first, to see if you can find some free info. :)
Here's a post that explains how to do it with an airliner, but it also has links to some ILS basics.
Here's a whole series of IFR tutorials you might check out (at a quick glance it looks like they use the default C172 in them).
This site is no longer being actively maintained.