Isn't the real question not "how" but "why"?

At least in the case of the general run of English "barbecues" - half raw half burnt hamburgers and sausages cooked by someone who doesn't know how to cook, using a contraption that fills the garden with evil-smelling, and probably very unhealthy smoke. Everyone lurches round the place with their burnt offerings, nowhere to sit, no tables for the food and drink...

The French do it better - cook the food in the kitchen and carry it out to tables with bottles, with chairs to sit on.

Or have a picnic with food prepared to be eaten cold.