The earliest record of the Plaid, or kilt, is about 1560. It was probably worn before this but there are no records of it. At one time it was argued that the C14 - C15 tombstones showing highland knights were plaids but it is now known that they are wearing padded armour with vertical seams giving the appearance of pleats. The garment was worn exclusively by Highlanders, and was not worn by the nobility except when at home on their estates. When at court they would have worn the same fashions as the southern nobles. There exists a letter written by an English nobleman who was invited to visit a Highland noble's estate for a hunting trip in which he describes the party halting on their journey to the estate to change into plaids because if they arrived in their normal attire the local guides would ensure that they didn't find any game while hunting. If they appeared dressed as locals then they would be led to plenty of game.
The plaid itself is basically a blanket. In period it would consist of two pieces of woollen cloth, each about 36" (90cm) wide sewn together to make a strip 72" (180cm) wide and between 3 and 6 yards (3 and 6 metres) long. I have been unable to find 36" wide fabric in an appropriate pattern and so my plaid is 60" wide (the standard width for modern woollen cloth).
The plaid would be worn over the other clothes. This did on occasion include trousers, or at least hose. It would usually be discarded before any physical exertion was indulged in as it was bulky and inconvenient to work or fight in. A long shirt at the very least would be worn underneath the plaid which would be tied between the legs, serving as underwear. A doublet, or coat, would also normally be worn as well as stockings, shoes, bonnet, etc.
To prepare the plaid for wearing first stretch it out flat. Then pleat it across the width (short side) so that it goes from 72" by 6 yards to 72" by about 1 yard. Lie down beside the folded plaid so that one end is above your head and the other is level with a point just above your knees. Period illustrations show the bottom edge being just below mid thigh - modern kilts are worn considerably longer due to Victorian modesty. (There are many 18th Century cartoons of soldiers in Highland regiments having their buttocks exposed by gusts of wind.) Pass a belt beneath the plaid, perpendicular to the pleats, level with your waist. Shuffle over onto the plaid (without disarranging it!) so that the belt is now beneath your waist and do it up, snugly but not as tight as possible. Pull the right edge across so that that it is on your left hip, then pull the left edge across to your right hip so that it is on the outside. Fiddle things so that it all lies flat and the bottom edges all line up. (This is easier if you can stand up and get an assistant to do it for you while you hold the upper part out of the way.) Tighten the belt as much as possible. If you now stand and let go of the top it should hang down to your ankles as a skirt. This is a perfectly period way for a man to wear it and keeps your legs warm.
If you want to walk about to any great extent you need to get the skirt out of the way. This could be (and was) done in a wide variety of ways. The easiest , but least practical way of doing it is to gather the top part into a loose bundle and drape it about your left arm. Very romantic, but not very practical. The best known technique is the forebear of the little tartan scarves worn on the shoulder in modern Highland dress. Take hold of the corner of the plaid and roll it back towards the middle, then do the same with the other side until you have a "tail" behind you, consisting of two rolls of fabric, one from each side of your body. Bring this tail over your left shoulder, across the front of your body and tuck the end of it firmly behind the belt at your right hip going between your shirt and the plaid. This should leave your arms and legs fairly free. I find that tying a piece of thong around the roll on my shoulder helps keep things in place. This also leaves a handy pouch of cloth around your waist to tuck things into. A variation of this is to take the fabric by the centre at the back and gather out towards the corners. This allows you to leave part of the corner hanging free. Instead of tucking into the waist the fabric can be pinned to the shoulder but with heavy fabric this tends to pull the jacket up at the front. The back can be brought up and draped over the shoulders or head as a cloak for warmth. Play about with it! Anything you can think of was probably done.
As for the type of fabric to use, bear in mind that the modern tartans are 19th Century inventions which bear only passing resemblance to their medeval period forebears. There was no "clan tartan" in the modern sense. Surviving portraits show clan chiefs wearing more than one pattern of fabric, frequently simultaneously (Plaid, jacket and waistcoat and hose all in wildly different designs!) none of which bear any resemblance to the modern "clan" tartan of their clan. The original idea of the "tartan" pattern was to use small amounts of wool dyed with the more expensive bright colours woven into a lot of wool which was either undyed or dyed with cheap, locally available dyes. This broadly means that the colours should be muted and natural looking. It would also mean that there would be a tendency for fabrics woven in the same area to have similar colours and possibly there were traditional patterns. Browns, greens and yellows would predominate with lines of reds and blues running through it. This would also be quite good camouflage while out hunting which may have a bearing on my opening story. This is born out by the surviving portraits. The bright red tartans are post period.
Finally, accessories. The Skean Dhu (small knife worn in the sock) is a modern affectation (& pretty useless). A dirk would have been worn (blade about a foot long) which frequently had a smaller knife plus a fork or pricker mounted on the scabbard. The sporran was a large pouch and not the sort of furry monstrosity commonly worn nowadays. As with any costume, try to have a look at a few period illustrations before starting your costume to see what was really worn.