You just put the piece you want on the appropriate square. If you wanted a third Bishop (say) you just grab one from an adjacent board (at home/or only one board you would put anything down if you don't have a Bishop (a pawn, an upside Rook {for a Queen usually} or a coin and declare the piece).
p.s. I've just had an idea for a puzzle. :)
i have never played a serious otb game (just fun games with friends) so i'm curious as to how promotions work.
do you just grab the piece you want, or do you have to declare it first, or what?