Sometimes the barrel will be held up by shoulder-straps, like a giant wooden pinafore. Other times it will seem to hover about your torso as if kept aloft by an invisible gust of wind from below, Marilyn Monroe-style.
Naturally, you will be nude beneath the barrel - that goes without saying. It's also likely that you will sport a waxed moustache or pipe or cloth cap of some description, although these details are to some extent negotiable. What is certain is that your nakedness will be accentuated by the comical addition of shoes - maybe even a pair of argyle socks with garters.
There may be a musical accompaniment. You might hear a jig or flourish of harps when you wear the barrel. Alternatively, there might be complete silence, as if all the air in the world just stopped moving. No one knows for sure which it will be. These things are determined on a case-by-case basis.