I’m not 100% sure what instruction you’re asking for. But since you said ILS approaches “had worked well until XP11 came out”, I assume you’re quite familiar with the technique of shooting an ILS approach, but just somehow could not get one started with XP11.
1. Start your C172 at KSEA airport. Open the XP11 Map and change its Mode to “IFR low enroute”
2. You’ll see a bunch of localizer courses depicted for the KSEA airport. Right click on one of those courses, a menu box will pop up describing the info for that ILS
3. Select the “ILS Approach” option
X-Plane will set up the ILS frequency automatically for you and start the airplane, flying in midair, on course for the localizer. You’ll just take over and fly.
That’s it. There is nothing to download. The ILS design in reality has not changed in decades, so there is nothing to make XP11 to be any different. Nothing new to learn.