Bed bugs have an unique scent. When you are a dog-EVERYTHING stinks uniquely.
A handler/trainer isolates/obtains the associated target. Then it is a matter of creating in the dog’s mind a ‘game’ that he scores points when he signals you (Sit, a bark, 3 legged stance like a pointing breed, etc.) whether the game depends on sight, sound or scent. Preferably you get to start the game—command it starts.
Dogs have instincts/traits that have been selected for (think of playing BALL with a Lab) and the training seeks to capitalize on, augment/expand, and communicate with the dog for a desired/directed outcome.
Its 2 way communication, listening to the hounds comes to mind. Watch a narco/explosive search dog....he sits hard and looks at the “quarry” when directed to ‘search’ or ‘find’ or released with an “OK”. A good handler tries to put his animal in target rich playing fields.
So-get a jar of bed bugs and carry on.....