Know Before You Go
The plaques are all located along the northern end of Trafalgar Square. Some are set into the wall, while others are embedded in the stairs leading toward the National Gallery. Your best bet for viewing the plaques is early in the morning or later in the evening, when there are fewer people crowding the square.
The measurements run the entire length from east to west on the steps across from the fountains. As of October 2019, the environmentalist group Extinction Rebellion have taken over Trafalgar Square, by setting up tents and putting on demonstrations. It is still possible to view the steps, just be aware of a large police and crowd presence.