Seeing as the belted plaid (aka great kilt, breacan feile, and feilidh-mhor) was an untailored garment, and there are many ways and styles of wearing it, it very well may look different each time you put it on. But with practice, you'll find it gets easier with time.

No offence to Scott, but what he describes is just a bit much, both in terms of fabric and the effort put into it. So let me describe a simpler and more accurate method.

Begin with your cloth (I recommend a medium-to-heavy weight wool, but since this garment is untailored and unpressed, you can get by with blends and other materials moreso than in a modern kilt). It should be at least wide enough to go from your knees to above your head (standard double width fabric usually works fine) and about 4 yards long (maybe as much as 5 for larger guys). Nine yards is entirely too much! The average total length of the belted plaids historically was about 4 to 4.5 yards.

Lay the plaid out on the floor. Kneel down at one end. You'll want to leave a length about half your waist size unpleated. From there, gather the rest of the material up into pleats. It's really better if you think of this as loose folding or gathering, rather than pleating. Don't attempt to pleat to a certain stripe, or to make each and every pleat the same size. Remember this is not a modern kilt! Believe me, the messier your pleats look the more "authentic" looking the end results will be. The goal is to have a gathered section that is also about half your waist size. Then at the other end of that, leave a length unpleated that is half your waist size again.

Now lay down on the pleated section of the plaid, with the bottom where you want it to fall on or above your knees. Take the right section of the plaid and wrap it around to your left. Then take the left section and pull it around to your right. Take your belt (you do have your belt handy, right?!) and run it under your back and then belt the whole thing securely in place.

Stand up. You'll now have all this extra upper portion hanging down over your belt. There are any number of ways to arrange this -- I'll just tell you one usual method. Take the front two corners and bring them around to the small of your back and tuck them into your belt. This will create two little "pouches" hanging in front of you, and also get the material out of the way allowing you to access your sporran, etc.

Grab the material hanging towards the middle (or just right of center) of your back and bring the edge up over your left shoulder (assuming you are right handed). Hold this in place with your left hand. Now reach underneath your left arm and grab the endge of the cloth that is hanging there and bring it up across the front of your shoulder and pin these two peices of cloth together (you do have a nice penannular brooch handy, don't you?!).

Viola! You are now wearing the great kilt. Like Scott said, it sounds complicated, but it is really easier to show someone than to tell them. Just ask if you have any questions. And remember there are lots of ways to do it!