
the way i did it (presuming they know the command all ready)
is to put a treat far away from me and attach my dog to a long line.
i show them the treat and walk with them a fair way away from it. i then let them go and they go off to get the treat but i stop them in time so they can't get it.
i then say down in a fairly loud voice ( so they get used to a shout if outside) and wait until they do it. i then click and make them come to me for the treat.
N.B never let them get the treat that you are using as the lure as otherwise you will get a dog that lies down and then runs off again to get what ever they wanted before.
once this is successful in the house take it outside.
HTH
anna
edited to say: i then move onto a ball or something being thrown for them to chase and calling a down in the middle (still on a long line to make sure of absolute accuracy.)
then the next stage is asking friends to ride their bikes past with them on a long line and continue with as many distractions as possible.
don't use the command off lead unless you are absolutely certain that they will do it or they wil learn to ignore it.