The thing to remember is that we are dealing, for the most part, with a length of untailored cloth that has been arranged around the body. Historic portraits show that it was worn in a variety of different ways. So rather than there being one "correct" way to wear the belted plaid, there were a multitude of ways.

The othe thing to keep in mind, as Rathdown mentions, is that we do not have anywhere written instructions saying how it was put on. This was part of their everyday life. It was something people just learned how to do and did, like us tying our shoes. There was no need for them to record how they did it.

So I doubt very much we'll ever get a definitive answer as to whether they put it on lying down, or standing up, or some other method. My hunch is the answer would have been "yes" to all of the above, and different people would have had different methods of putting it on at different times.

In my own experience doing both reenactments as well as educational demonstrations, I've put on the belted plaid both lying down and standing. I am able to achieve a neater and more controlled look when I do it lying down. But sometimes the circumstances of where I was getting dressed made standing up a better choice. I expect the historic highlander would likely have used different methods based on circumstances, as well.

But for the modern day reenactor, as we don't have written directions from the period, all we have to go on are historic portraits showing it being worn. So any way that you can arrange the plaid so that the end result looks like the portrait or portraits you are going by is a legitimate method.