Two words, no agreed definition
Grounding and centring are used constantly in modern magical writing and almost never defined. Some practitioners mean something literal: a subtle force that can be sensed and directed. Others mean attention, nervous system and mood, described in metaphorical language. Plenty hold both loosely at once. None of those positions is the beginner mistake. Assuming everyone means what you mean is.
Grounding, practically
Grounding usually describes coming back into your body and your surroundings: before ritual so you are actually present, and afterwards so you don't drift through the rest of the evening. The classic methods are physical. Feet flat on the floor, slow breathing, cold water, food, going outside, touching something textured.
Centring
Centring is gathering your attention into one place before you begin, usually described as settling into the middle of yourself. Read literally or metaphorically, the practice is the same: a deliberate pause in which you decide what you are carrying into a room.
A sequence that fits in a corridor
Not a traditional rite. Before something difficult, stand still for thirty seconds, feel your feet, take three slow breaths, and name silently one thing you are not taking in with you. Afterwards, drink something and notice the room.