By far the most common form of slippering was to be called to the front of the class and told to bend over. (I don’t remember any special instructions like legs apart or touch your toes. Provided you bent and pointed your bottom in the right direction, this would do 
) The teacher would then give (almost always) three strokes to your bottom.
The slipper was a gym shoe, usually a plimsole, but sometimes something more like a modern trainer. I never saw a leather carpet slipper used.
The technique used by most teachers was to give one very hard whack, and them a long pause…maybe 10 seconds…and then give two more hard strokes, straight after each other. Everyone agreed that being made to wait for the second whack was the nastiest part of the punishment.