Can someone advise me to a step-by-step instruction of flying and landing on autopilot, please?

I've been going through loads of videos on youtube, but its all overloaded with extra info and mostly for way more advanced planes then that are relevant when learning to fly. The ones that I can follow dont get the plane on the ground, it either flies over his target, or it turnes around and flies... I dont know where, it just keep flying straight out untill I quite the program.
Also, one time when I switch to approach, ALT stays on and goes off once the plane hits the glideslope, but another time it goes off when I switch to approach and the plane dives into the ground and when I switch it back on manually, the plane flies over the target again. And it seems that every time I perform the same procedure, something different happens...

I get it done to fly from one airport, to the one right next to it, by setting the ILS radiofreq to NAV1, go to APR when the plane starts beeping. But flying the same route back, it goes in holding over the airport I fly to and just keeps circling and I cant get it down again. Also, soon as I have to fly a longer route by GPS, I cant get the plane to land either way.

just a simple step-by-step procedure of the actions I have to perform on the standard Cessna 172, from setting up my gps route, untill the plane landed on the desired runway: push this button, that will happen. Push that button, this will happen...

Thanks

If in it looks like its about to miss,ignore ILS.

It's not as simple as step 1, 2, 3, unfortunately.  Using the autopilot to fly and land isn't hard, mind, but it require you to understand: a) how to set up an approach; b) how the GPS works in FSX; c) how the autopilot works in FSX; d) how ILSs work. 

One thing to keep in mind is that, unless your flying an advanced airliner like the PMDG 737, that has full-functional VNAV and LNAV (vertical and lateral navigation) capabilities, your autopilot won't do all the flying for you.  The autopilot will certainly help, but it won't be as simple as programming a route, and getting you from takeoff, cruise, to the full autolanding without your input.  In fact, I don't believe you can autoland in FSX unless your flying an aircraft that is designed to do so (PMDG, level-d, Captain-sim, those kinds of products).  Attempting to do so without the capabilities would result in crash, nose wheel landings, or hard landings!

Firstly, let's talk about the ILS.  The ILS is a handy system that will tell you both your glide information and whether or not you are on the centerline of the localizer (usually the center line of the runway).  This information is displayed either on your HSI, or a separate ILS instrument. 

Normally, when you approach an ILS, you want to approach the localizer at less than a 30 degree angle.  You also want to be below the glideslope.  The order of which things should happen is that you intercept the localizer first, then, as the glide slope indicator begins to come down, that is when you start your descent.  Remember, be below the glide slope and try to hit the localizer first.  If you are using an autopilot for this, you would normally be set up at an angle towards the localizer.  You would then hit either the NAV or LOC keys on the autopilot to intercept the localizer.  Once the aircraft has intercepted the localizer, you should hit the APP key to intercept the glideslope.  Remember, you need to have the ILS frequency tuned into your NAV radio, and have the correct runway course tuned into your ILS instrument (course and frequency can usually be found in the map by clicking on the airport you are landing at). 

Now, let's briefly talk about the FSX GPS.  The FSX GPS is generally a more simplistic version of the real garmins out there.  It has it's limitations, but for the most part, it's a pretty functional device.  In FSX, there are two modes: NAV and GPS.  Whenever you are navigating on a GPS track, on autopilot, you must be on GPS mode before the autopilot would do anything to follow a particular track.  When you are navigating by radios, such as by VORs and even the ILS, you have to be in NAV mode in order for it to work. 

The programming of waypoints in the FSX GPS is done solely via the use of the flight planner.  I don't think you can do things on the fly, unlike in real life, so that is one of the more notable limitations of FSX software.  Therefore, one of the things to consider is that the FSX GPS will not get you set up and lined up for an approach as easily as one might think.  You almost have to approach the runway a certain way, and even then, the only way to get lined up is by pulling up say an RNAV (area navigation) approach by pressing the proc key and going through the menus.  Therefore, you have to fly the aircraft.  If you are using the autopilot, this is where heading select or heading hold becomes useful.  Otherwise, you can hand fly the airplane.  The idea is to position yourself for the approach, because the autopilot will not readily do this for you.

Realistically, you won't be using the GPS much to do landing procedures in most cases unless you are specifically shooting an RNAV approach or for doodling around.  You have to set up your own approaches.  Either the ATC will tell you how to set this up, or you would follow the instructions on an approach plate, chart, or even just land conventionally via joining downwind and landing by visual reference to the ground.

But as you are curious about the autopilot, the best way to understand this device is to understand what the features do and to experiment with how they work.  Try playing around with the heading holds, the heading selects, the altitude selects, vs for adjusting your rate of climbs and descents, and the nav function, which can be coupled to either a selected VOR course or GPS track depending on whether you are on GPS or NAV mode. 

My best advice to you, assuming you know the basics of at least how to fly a sim airplane, is to plan a simple IFR flight plan in FSX from one airport to another.  IFR in aviation is generally a bit more complex than VFR in many ways, but in FSX, you can get away with a lot in terms of making mistakes and learning and is also quite straightforward even for the non-real-world-pilot.  You also don't have to memorize numbers such as weather minimas and airlaw stuff like in real life. 

File an IFR flight plan, then use the default ATC in FSX.  Reason why you're using IFR is because ATC will pretty much tell you what to do, such as what altitudes to fly at, what headings to fly to, and who to contact.  It's a great way to experiment using the various features on your autopilot, and there is not much repercussions for making a mistake.  ATC will also vector you and descend you down to a position that sets you up for an approach, whether that be ILS, visual, or otherwise.  It's a great way to see how approaching an airport can be like, and while it isn't totally realistic compared to real life, it's a good start to understanding the basics.

This is what I've done now:
I started a flight in Skavsta, Stockholm (ESKN) to Arlanda (ESSA). I programmed a FPL in the flightplanner, programmed an approach at runway 01R, set the freq of that runway to my NAV1 radio and the heading of 007 on my NAV1 OBS.

I took off manually and once in the air I had the autopilot take over in GPS-mode and the aircraft lined up with the route shown on my GPS.
When my runway came in sight on the GPS screen I activated approach to have the aircraft line up roughly, which also worked out fine.
When I got in range of the radio freq and the NAV1 started beeping, I turned off NAV on the autopilot and took over the lining up manually, lined up well within the 30 degree angel over the runway direction, switched to NAV-mode and turned on APP on the autopilot to pick up on the ILS.
The aircraft seemed to line up but turned a little too far and ran of course again, so far that it drifted over the 01L runway to the side.
When I switched the NAV/GPS mode switch to GPS and immediatly back to NAV and it corrected its course and made a smooth landing.

So again the aircraft as well as the program show it can perform all actions I need, but it again shows a weird bug at some point...

The reason I tried to perform autolandings with those small aircrafts is that I wanted to master the procedures on basic aircrafts, just for practice, before getting into the bigger aircrafts where I need to control a lot more instruments, but I think I"m gonna have a go with that now.

The guy from FSX support at Microsoft made it seem I should be able to perform the same procedures with every aircraft and had me do so many variations of the same procedures that I couldnt see the forest throught he trees anymore, it seems.
I tried again and simply turned the autopilot off before switching from GPS to NAV and turned it back on and now it seems to work fine. Tried both Cessna 172 and Mooney Bravo on a couple of flights and as long as I stay inside that 30 degree angle, that aircrafts touches down perfectly.

Thanks a lot for your help, I understand a lot more of whats happening and where I got stuck. I'm glad I got over this issue and can move on now.