Pegler Terrier is a very high-quality manual valve; and it is even possible to buy repair sealing rings when they wear out (after about 30 years)
To turn the off, you simply screw down all the way until you shut.
To open, you simple screw upwards a few turns (never wind valves hard against the stop or they will seize)
If you want a alve to be around a quarter or a half power, first wind it fully shut (gently) and leave it for half an hour or more. When you come back, the radiator, and both the conected pipes, should be cold. Open it half a turn only and do away for another half hour. When you come back, one of the pipes will probably be hot, and the radiator will probably be somewhat warm. It will be hotter at the top than the bottom. If not, open it another quarter turn and give it another half hour. The pipe that heats up first is the Flow pipe, and the other is the Return pipe. Mark them indelibly with an "F" and an "R" because one day you will need to know.
Because of the way valves work, almost all the fine adjustment will be around the first turn or so from fully closed. There is no point in winding them to what you think is fully open.
Once it is open enough for the rad to become somewhat warm, leave it for a day and decide if the room is about as warm as you want. From now on, do not adjust it more than a quarter turn per day.
Write down how much you have opened it. Sooner or later somebody will interfere with it and you will want to go back to your adjusted setting.
If you have rooms that constantly get hotter or colder than the others, you can balance your entire system. This is not done by opening up the cold ones, it is done by closing down the hot ones.