Keep in mind that the word plaid comes from the Gaelic for "blanket." The original plaid was simply that -- a large woolen blanket worn wrapped around the shoulders in a cloak-like fashion to keep warm and dry. At the very end of the sixteenth century, the plaid began to be worn belted by the men. At this time, they were made from two lengths of cloth, some 4 or 5 yards long and 25" to 30" wide, seamed together to make one peice that was 50" to 60" wide. The length was gathered into folds and belted around the waist. The part that hung from the waist to the knee would later develop into the kilt. The upper portion was worn draped in various ways about the shoulders. This garment was called the feileadh-mhor (large wrap) or "belted plaid."
At some point in the late seventeenth century, or early eighteenth century, the two peices of the belted plaid began to be worn seperately. That is, the lower part was a 4 yard (on average) length of tartan some 25" wide, still worn gathered up and belted at the waist -- the feileadh-beag. The upper length of cloth was still worn around the shoulders in a plaid-like fashion, though it could now be easily removed indoors with the wearer remaining dressed from the waist down.
In the era of the modern, tailored kilt, we have come up with various styles of plaids that are meant to emulate the upper portion of the old belted plaid. The simplest, and most appropriate for general day wear, is not often seen today any longer. And this is just a pure and simply plaid -- that is, blanket -- sometimes called a shoulder plaid. Get a length of tartan cloth -- the exact deminsions don't really matter, but keep in mind that plaid means "blanket" so you will want a couple of yards at least. If you like the ends can be finished with a fringe of some sort. This is just worn folded up and draped across one shoulder, like you would carry a blanket to a picnic. If you want, to keep warm in cold weather, it can be unfolded and worn as a cloak, like the original plaids were.
What is more commonly seen is the "fly plaid." This is usually reserved for formal wear. There are a few different styles of this, but it is basically a peice of tartan cloth some 54" square (or something around that size). The edges are usually fringed. Often one corner is tailored into pleats, and it is this corner that you wear pinned to one shoulder with a large brooch. The remainder is left to hang free in back. One style of this has a tie at the lower end of the plaid that attaches it to your waist, making the garment, when worn, look more like the upper part of the old belted plaid.
And then we have the pipers and drummers plaids. These garments are essentially the same except one has more cloth than the other. Basically this is a length of tartan cloth some 3 to 5 yards long, with the width of the cloth pleated up and tailored down to something like 12 to 16". The two ends are fringed. This is worn over the doublet and all, across the chest and down the back, usually only by those in bands, sometimes just the pipe major and drum major.
What you want to avoid is a lady's sash. Sometimes those unfamiliar with Highland Dress will have this vague notion that something tartan should be seen on the man's shoulder and they buy a woman's sash for the purpose. But as women's sashes are only some 10" to 12" wide and usually 90" long, and most always made from a lighter weight cloth than a kilt would be, you can see how they would not compare to any of the garments described above.
Now, to confuse the issue even more, some retailers have of late introduced items, for men, called things like "mini-fly plaids" or "fun plaids" that are touted as fly plaids for men, but for more casual wear. These that I have seen have roughtly the same deminsions as a woman's sash, and in my opinion fall in the same category and should be avoided.
As a final note abbout the lady's sash -- the old adage about what shoulder she wears it on denoting rank in the clan is a myth. If you are right handed, it should be worn on the left shoulder, and vice versa. Why? Because it's more practical that way!