How do they calibrate it?

A tog value is a peculiarly British measure of thermal insulation where 1 tog is equal to -

"ten times the temperature difference (in °C) between its two surfaces when the flow of heat is equal to one watt per square metre.".

This is such a complex statement that it was simplified to a "TOG" - which was invented after WW2 in Manchester university - where the slang word for clothes is "togs"

Actually, I'd never heard of it, despite living in the UK after duvets were invented.

Must be similar to "R" value used here (Canada, at least) for house insulation.

The web says that one tog is about 0.1 m^2K/W and is measured by a togometer.

Generally, instruments are calibrated against samples of known characteristics, or against other better calibrated instruments e.g. by being sent away to a standards lab. Eventually the standards labs refer everything back to fundamental constants like the standard kilogram and the speed of light. It gets complicated and is quite an involved, but important, process.