As an old D/F operator, it goes something like this:
Pilot requests a 'steer'.
You tell him to transmit continously for 10 or more seconds.
You rotate the antenna until you reach a null point, i.e., no sound.
Verify if a TO or FROM heading between him and your location.
Advise that his bearing is TO or FROM you accordingly.
Not to boast, but I had one occasion where an aircraft was out over the North Atlantic and due to IMC became totally lost. I gave him a steer to my station but wasn't much help since he wasn't headed there and the distance between us was unknown and he couldn't pick up any other stations for a fix. A triangulation was needed for that. Out of nowhere, I came up with the idea to try something. I had him turn TO my location, then turn 90 degrees, fly 5 minutes at a constant airspeed,, do a 180, fly another 5 minutes at the same exact airspeed. Meanwhile, I was taking bearings from his transmissions at precise times as I requested. As a result, by simple triangulation, I was able to tell him almost exactly where he was.