There are rare occasions that a plaid is acceptable - basically for court dress (i.e., if one is attending the Royal Court) and for the groom at a wedding. In almost every other circumstance, it is overkill and inconvenient.

If you must wear one, though, I think the "half-belted plaid" that Matt Newsome wears is the best way to go. Here is a link to his blog post about it.

As for the shoulder thing, that is really for women wearing sashes. A clanswoman wears her tartan sash pinned on the right shoulder with the loose ends hanging down from there; a woman who has left her clan through marriage but still wishes to wear her tartan wears the sash pinned to the right shoulder but with the loose ends tied in a bow on the left hip; the wives of clan chiefs, chieftanesses, and the wives of colonels of Scottish regiments wear the sash pinned to the left shoulder with the ends hanging down from there; all of them may wear the sash pinned to the right shoulder and draped down and fastened or belted at the back.

Probably more than you wanted to know...