I found out, due to reaction on my tutorials, that they seem to work. But ... I still did forget one part. I teached you how to make your model. But now it is time to prepare it for FMS. This is how I do it.
If I tell you than normally the F1 button gives you the SIDEview, you know that you are in trouble. You have always used F2 for sideview, right. Ok, my system uses the squares to help you draw the sideview, but normally they are used to see the topview. Lets get it all on the right place. Easy!
All works better if you make the mirrored object not visible. So ... hit at the top of the screen the symbol SI till the mirrors disappear.

step 1: make ALL object visible and unlocked!
step 2: select ALL objects.
step 3: click in the left menu ROT, a small menu will appear. Click NUMBER.

step 4: click the middle square with the rotation around the green axis. FILL X, Y, Z WITH ZEROS! DON'T FORGET IT!
step 5: put in angle -90. Look good! MINUS!
step 6: do the same but with the blue axis and with 90°. POSITIVE!
OK, you have it turned properly.
But, if you now go click SI again, you will see that the mirrors fall badly. OK, easy to solve. Just select a object and go to its properties. Change mirror from Y to X. Done. OK, repeat that with ALL your mirrored objects.
The first basic change is done.
Next large step is to get the model in a good size. If you would know that most of my models have a span of about 4 squares, you will see that you have some serious scaling to do. How do it? Just follow.
step 1: Make sure you still have ALL the objects selected.
step 2: Go place the view in topview. You should now see the squares.
step 3: Hit SCALE in the left menu. A menu appears. Hit NUMBER again.
step 4: fill again X, Y and Z of CENTER with zeros. DON'T FORGET IT!
step 5: go count the amount of squares you have at this moment for your span. calculate roughly the number you need to get the model at span of 4 squares. How can you do that? Divide 1 with the amount of squares you have for span at this moment. Multiply the result with 4. That number is the number you need. Change the X, Y and Z of SCALING to that number.
step 6: most difficult step. ... Hit APPLY. Done.
OK, your model is ready to be saved as X-file. So, click in the menu at the top FILE. Select SAVE AS. Change the type of file to X-file. Give it a name and a place to be saved on. After hit SAVE, another menu will appear. Here you change DIGIT to 1. And I always use MULTIPLY as 0.0020 . Experiment a bit with the selectable things there. You will see that the shape or shading can diver if you choose NORMAL MAPPING or UV MAPPING.
You are nearly ready. You have your own x-file of your model. Now you still need a sound and a PAR-file. Editing sounds or PAR-files is still not one of my specialties. I admit. I even stink in it. Pardon the language. But ... just do like me. Choose the easy way. Copy the sound and par of a airplane that looks similar to what you intended to fly. If you are making a glider, choose a glider. A jet for a jet. A slow flying airplane for a slow flyer. And so on. All you need to do is copy them and give them a new name. Exactly the same name as your X-file. Of course ... don't change anything behind the dot.
You are now able to fly your model. Go start FMS, load your model and test-fly it. If the model you copied flew well, this one will fly identical. Counts too if the original model flew bad!
OK, last item. The image as preview. OK, fly your airplane and place it near something you like to see in the background. Pit it on PAUSE. Use the numeric keyboard to turn around the model and to zoom. Choose a good view and hit SHIFT and PRINT SCREEN on your keyboard. All you need now is a image-editor. Open it. Select file and choose PASTE or OPEN FROM CLIPBOARD. You will get a image of your screen. Now it is up to you to make a image of 200 x 200 pixels with a good view on your model. Save it next to your model files with the same name as your model but with "_prv" behind it. Voila. Finished!
Well, that is how I did it to create my own models. Might be you will find a shorter way to do it. If you do, please, tell me. I will place your remarks here and it might help the next viewers.
Enjoy FMS and its model-making.